It might be open source, or it might not be, but eventually, someone will come along with a "better Facebook than Facebook", and it will slowly die.
That's just creative destruction at work. It ALWAYS happens.
Facebook was a better MySpace than MySpace.
MySpace was a better Friendster than Friendster.
Friendster was a better Classmates.com than Classmates.com.
Google was a better Altavista than Altavista.
AOL Instant Messenger was a better ICQ than ICQ.
USENET was a better BBS than old-school dialup bulletin board.
Books were better scrolls than scrolls.
Something newer and better is going to come along. People talk about Facebook and the network effect "locking in" people, but creative destruction is even more powerful than the network effect.