Maria Konnokova for The New Yorker:

While the reasons for joining and using Facebook were not entirely homogenous, one factor kept emerging as the strongest motivation for use: the desire to keep in touch with friends.

I think most people would agree with this.

At the University of Texas at Austin, Gosling and one of his graduate students, Gabriella Harari, have been examining why people decide to leave Facebook. They have found three broad themes: people see Facebook as pointless and unnecessary, they see it as a problematic distraction, and they are worried about privacy.

So, the first two reasons people quit Facebook is also the reason they joined in the first place. Interesting.