Addicted to the people's drug internet?Relatively early, studies emerged that pointed to the dangers of internet use, such as the disruption of social relationships and threats to psychological well-being - both paradoxically triggered precisely by virtual human exchange via the internet (internet paradox).  Subsequently, much research was conducted into the internet and its various forms. It turned out that there was no simple formula "the more internet consumption, the more depressed". Too many factors in the individuals and in their environment had to be taken into account.
Nevertheless, the addiction-narrative is a constant companion in the discussion. "Drugs" dealt include the internet itself ('internet addiction','online addiction'), (online) shopping addiction, (online) gambling addiction, the addiction to online pornography and, most recently, social media addiction.  There are even handy online tests where you can determine your addiction level. 
So is the internet successful because we have become addicts of a people's drug ? I don't think so. Whether internet addiction can be classified as a disease is still controversial. However, gaming disorder has already made it into the WHO classification system, probably because it already contained gambling disorder.  And the medium's success is hardly explained in terms of addiction. Alcohol, for example, is not so widespread only because it is addictive. However, it is true that it is not always easy to control the use of the internet and social media. Remember the last time you tried to watch "just this one video" or read "just this one news".
Victims of our evolution!Another explanation, perhaps even more disturbing, is that we can't help but being 'social'. The human brain has a social default mode. It is - from an evolutionary view point - in our nature to connect with other people: Comparatively immaturely born, human babies could only survive because there was a strong parent-child bond. Adults could only survive because they were part of a group.
Much of what constitutes an individual has to do with other people: In the course of our lives we learn from others. We do not only learn how to imitate their behaviour. We also learn to understand the behaviour of others because we can take their perspective. Through socialization we adopt the standards of our group, which become part of our self-image. Self-control gives us the ability to live in harmony with the group and to contribute to it. Pain caused by social events (separation, death of loved ones, bullying, etc.) feels as real as pain caused by physical impact.  Our urge to be in agreement with the group leads so far that we think and act against our better knowledge in order to remain in harmony with our group. 
The social media take advantage of this human disposition which makes us exploitable.
About features and exploitsJust as computer software offering exploits, our brain is vulnerable to the features of the apps on our smartphones. Here is a small selection of exploits and features that keep us busy with the internet and above all with social media.
The social default mode of the brainWhen we are idle and have nothing to do that requires our attention and mental strength, the brain does not take a break. Some regions remain active. Remarkably, these are exactly the regions that are activated when researchers study social cognitions. This social pre-setting of the brain is explained by the fact that it was evolutionarily crucial for the social success of humans in the group to interpret the behaviour of the other group members correctly at all times. 
That's why so many people immediately pull out their mobile phones during periods of mental rest. Social pre-setting, curiosity and play instinct go hand in hand. What's new on my social network? Who is currently available for an online game? Thanks to our mobile phones, we carry "anyone anytime, anywhere" with us and are almost always "on". And to keep it that way, the apps offer contact suggestions, invite our contacts to the app or send invitations to our (previously read) contacts without our assistance.
The need for belongingBeing part of it is a fundamental human need. We need a minimum of lasting, positive and meaningful interpersonal relations. The feeling of belonging is decisive for our well-being. Isolation or even exclusion have serious physical and psychological consequences. 
This need is also satisfied. In online communities like-minded people come together, photo enthusiasts e.g. at flickr. People with the same condition (e.g. a serious illness) help each other in online groups. And dating communities offer the possibility to find a romantic partner.
The rule of reciprocityReciprocity describes urge to return a favor. This strong impulse is universal and had - from an evolutionary view point - probably an advantage. If you helped a group member out of a jam you could expect her or him to help you next time when you were in trouble (reciprocal altruism). Free riders, on the other hand, had to reckon with disapproval by the group or even exclusion from the group. The impulse for reciprocity is so strong that it can be used against us, e.g. in a sales situation where the seller imposes a free sample on us hoping that we will then buy the product. 
Reciprocity too plays an important role in the social media. In order to expand reach e.g. social media managers follow other participants on Twitter, hoping that they will follow back. And what do we do if we receive a contact request on Facebook or Xing/LinkedIn from someone whom we hardly know? We will probably accept it.
Status and Social RecognitionThe position in the group is highly relevant for the individual because it determines its access to the resources of the group. Accordingly, we are sensitive to signals that allow us to recognize our own social position and send out signals that indicate our own position. 
On the internet, this is made possible by interaction mechanisms such as likes and comments. Whoever posts something hopes for appreciative feedback from fans or followers and is disappointed or frustrated if it fails to materialize. In the comments under some posts battles are raging in which the participants are no longer concerned with the controversial topic, but with the disparagement of the opponent or the opposing group.
Conformity and confirmation biasTwo other weaknesses may not be directly tapped by social media features, but they do have an impact on behavior in the social media.
Conformity describes the effort to remain in accordance with the group's standards of thought and conduct.  The pursuit of conformity not only ensures that we rarely oppose the opinion of our own group openly. It may even lead us to stick to the wrong group opinion, despite the obvious evidence to the contrary. The followers of Donald Trump are a particularly blatant example. They were shown pictures of the inauguration of Trump and Obama. 12% of the Trump supporters described the crowd in the picture of Trump's inauguration as larger, even though this was in stark contradiction to the actual proportions on the pictures presented.  People who see so much what they believe are no longer accessible for factual arguments. The social media conformity feature par excellence is the "Like". To my knowledge only youtube has a dislike possibility.
The confirmation error describes the tendency to look only for information that confirms one's own view (or that of one's own group) and to hide or devalue contradictory information. In this way, the individual succeeds in reducing cognitive dissonances that are triggered by the contradiction.  Along with the tendency to conformity, the confirmation error ensures that group members immunize against conflicting information. The urge to eliminate cognitive dissonances goes so far that individual group members try to convert people with contrary opinions in order to find confirmation for the group opinion.
To the point
The internet has undergone a remarkable development in a short time. From around the 1990s onwards, it became accessible to a wider public as a medium for information and its exchange. Commercialization through sales platforms also began in the 1990s. Blogs and virtual communities made the internet more personal before it finally became 'social'. This development was supported by the ubiquity of the smartphone. And - in hindsight - this development was foreseeable; for the social internet is an expression of the social human nature. With ever better features, the platforms have managed to exploit our social "blind spots" and made themselves indispensable.
 Nimrod, G. (2013). Challenging the Internet Paradox: Online Depression Communities and Well-Being. International Journal of Internet Science, 8(1). Link
 Gesundheitsstadt Berlin (2018). Wie süchtig machen soziale Medien? (DAK-Studie). Link
 Z Internet Addiction Test. Link
 Füller, C. (2016). Die neue Volksdroge. Der Freitag 25/2016. Link
 WHO (2018). Gaming disorder. Link
 Lieberman, M. D. (2013). Social: Why our brains are wired to connect. OUP Oxford.
 Asch, S. E. (1956). Studies of independence and conformity: A minority of one against a unanimous majority. Psychological monographs: General and applied, 70(9), 1. Link
 Baumeister R. F., & Leary M. R. (1995). The need to belong: Desire for interpersonal attachments as a fundamental human motivation. Psychological Bulletin, 117, 497-529. Link
 Cialdini, R. B. (2007). Influence: The psychology of persuasion. New York: Collins. Link
 Van Vugt, M., & Tybur, J. M. (2015). The evolutionary foundations of hierarchy: Status, dominance, prestige, and leadership. The handbook of evolutionary psychology (2nd ed.). Hoboken, NJ: Wiley.
 Schaffner, B. & Luks, S. (2017). This is what Trump voters said when asked to compare his inauguration crowd with Obama’s. The Washington Post. Link
 Tavris, C., & Aronson, E. (2008). Mistakes were made (but not by me): Why we justify foolish beliefs, bad decisions, and hurtful acts. Houghton Mifflin Harcourt. Link