I am not a fan of Facebook, but lets think about what you said. What other site has risen to the level of popularity that Facebook has? And have those sites disappeared or lost popularity? We can start with Yahoo I suppose. Still huge, still around, and a completely different set of services offered. Geocities? eh, maybe but they were only popular with a segment of the internet population at a time when being on the internet was not cool. (Plus I never cared for Geocitie's pages) MySpace? While certainly their status has declined, hey are still kicking. Plus, Facebook actually pre-existed MySpace, then experienced a decline to MySpace, and has now far surpassed MySpace.
I would argue that AIM and ICQ are the closest analogies. Except that Facebook replaced them. Those services were designed to connect people and now Facebook does that (arguably better). Sure some people still use those services, but a lot of people just use Facebook for those things.
I guess my point is, please name the flash-in-the-pan popular sites you are referring to that have reached Facebook's level of pervasiveness in society? I only ask, because I have been on the net since '90 and cannot think of another and I am really trying. I am not saying there isn't a site out there that I am forgetting or that there isn't a site that I never knew about, but I would be really surprised.
Slashdot has been around a long time and has a dedicated following, is it a flash-in-the-pan popular site? I mean, /. is really a place for people who were/are on the forefront of the emergence of the Net into our lives. Can anyone think of any single site that crosses more culture, economic, or age brackets? Say what you want, but they did something right. And picking themselves up from their bootstraps to comeback from near-defeat at the hands of MySpace is something to be respected.