Well I'm another FB user.
I joined back in 2008 but never did anything much with it. A friend of mine setup a group for a local piano museum, and asked me to joint so I did. There it sat untouched until mid-2010. I was an older student then at UMASS-Lowell, and a bunch of students and faculty were up there that I used to hang out with. We used this for conversing over projects and such. At the same time, a friend of mine, a storm chaser and photographer, was using this for his photo blogs and essays. Thus, I became a Facebook user. If however, the site disappeared I could care less. The piano museum still exists, and has for the past 30-plus years. My storm chasing group that I go chasing with, has their own website anyway, so there's nothing amiss should I not bother to visit FB.
In addition to FB, I am also a member at some other tech forums, and over at a couple of medical-related ones. With one of the medical forums, I became a technical lead for a book that some members are writing. I am also over at a virtual railroad forum at www.auran.com where I help with tech support issues.
I admit though it is a nice way to keep in touch with former coworkers that have long retired from the workforce as well as family and friends that now live many thousands of miles away.
I tried twitter once. What turned me off were the stupid posts like announcing to the world that someone had to use the bathroom. Who cares about mundane stuff like that.