I think Facebook is a lot more defensible than Google. I think you seriously underestimate how inertial the relationships it now harbors are. Someone just has to build a better, cheaper, Google, and it will slowly start to leak market share. It's a lot harder for Facebook to spring a leak, because I'm not interested in jumping onto something where only 10% of my friends are going to follow me.
Facebook is the first thing, the ONLY thing, that's gotten hundreds of people I know (I mean, really know) to all be in the same "place" at once.
I abandoned Friendster and MySpace long before Facebook showed up. Neither of them ever truly reached the critical mass that Facebook did IMHO. If you remember what Friendster and MySpace were like in earlier days, you kind of just friended people and sat there. The engagement on Facebook is MUCH higher.