Rangan.com is an excellent blog that deals with communications and public relations for businesses. In a recent article they had an infographic on why people use social media. Basically, usage of social media produces the same dopamins as sex and food. The article also states that people would rather give up sleep, alcohol, cigarettes and sex in order to keep their social media. Strikes me as bizarre that social media is more popular than sleep or sex. Maybe that's why the Y generation is not getting married:)
While the biology may explain the phenomenon, what triggers the behavior of social media? I think the explanation goes back to early child behavior. Children are able to teach at age two before they can read and do many other things. Social media is a form of teaching, perhaps through showing off. Why do so many people repeat the same quotes from Ghandi on Twitter? Everybody wants naturally to teach.
The second reason everybody uses social media is that it enhances self-esteem. Social media is the adult version of presenting a finger painting to a parent for approval. I believe that much of computer usage in general relates back to self-esteem. Computers make output very easy and creative and artistic output much easier, which increases the opportunity for self-esteem building outputs dramatically. This blog might be evidence. One might also cite Github, the open source coding forum, as another example.
I think more research is needed to show the relationship between self-esteem, computers and social media, but there is more to learn than just the biology.