Social media sites are the collective equivalent of a "dollar auction"[1]
In the dollar auction, each immediate choice isrational, but the overall process inevitably results in you losing value once you've decided to participate. But at each step, you lose more by quitting than by keeping going.
Social media is the same, except that it's collective: if you want to keep in touch with certain people, you don't have a choice other than to use the social media site they are using, so leaving is a collective action problem.