The brilliance of Facebook is that it draws in lots of users. Their funnel is amazing. The app at the bottom of that funnel isn't that great, but manages to deliver value because of the huge userbase.
And this is also its weakness because if its only virtue is that it's "the cool place to hang out in cyberspace" then this is very vulnerable to changes in fashion (see MySpace/Second Life/LiveJournal/GeoCities).
Except at this point your parents are on Facebook. They were never on MySpace/Second Life/LiveJournal/GeoCities. While some of its value is "the cool place", it's also the place a lot of my family's social events (birthday parties, etc) are announced. If my 23 year old cousin switches to HotNewSocialSite, she'd still have to be on Facebook to find out about Granddad's 90th birthday get together simply because that's how the word gets out now, for better or worse.