The driving force for leaving AOL was clear: there's a big, exciting internet out there that's more interesting and innovative than AOL chatrooms and AOL keywords, and the user base was also getting a little more comfortable with computers and didn't need AOL to hold it's hand so much any more.
I don't see a similar driving force for leaving Facebook. As a matter of fact, Facebook is embracing innovation, pre-emptively embracing whatever things might give users a reason to jump ship.