My bet is that Facebook doesn't die off, and it's because Facebook captures what really matters for social networking: keeping track of friends. That's why it beat MySpace. Before Facebook, no social network was good. Each one was a bit less shitty than the one before it. Facebook's Feed was what pushed Facebook over the top, and it's powerful enough that Facebook won't lose what it's got. Facebook might waver in popularity, but at its core it's so good that people have no reason to switch, whereas with every other site there was no reason to stay once something better came along.
Among my contacts, the only ones that use Twitter are the publicity hounds. The ones who want to keep in touch use Facebook and Facebook statuses, and because statuses are more interactive than Tweets (comments and likes), that's the sticky part of my communications world.
I don't get the people calling Facebook a fad. Perhaps it's only coming from people that aren't part of the Facebook generation? Looking at it from within, as I'm doing, the difference between this and MySpace/Livejournal/Friendster is enormous.
Geocities wasn't social networking, it was free web hosting for people who didn't care about having their own domain name. LiveJournal is a quasi-social blogging platform, MySpace never sustainably spread past a teenage/preteen/music niche, and Facebook/Twitter are complementary services (Facebook is privacy oriented and Twitter is publicity oriented).
