I was thinking about the allure of Facebook. Previously I attribute it's dominance to a combination of "stream/wall" (ala Twitter, Tumblr, Instagram) with real identities, a strong bias to guide you to connect with people you know in real life vs anonymous internet strangers. But there's something else: the "also commented on X's post" notification. I just now posted a second comment on friend's post on feeling blue and the seemingly masochistic desire to watch or listen to sad stuff there, two passages relevant to the topic (this passage and the Mr. Blue thing it links to) and it feels great that I know people interested in that topic will get a nudge and probably see what I put there. Old school web forums have this feature but they don't have the stream that brings content front and center, or the "real world friend" aspect.
(Heh, I remember when I would read Usenet on an old academic account, I had a perl script that would scrape and find me continuations of threads I had participated in.)