Nothing lasts forever, look at MySpace. It was so amazing, and now it is nothing compared to Facebook, if not at least when it comes to the userbase. So, What will make Facebook a memory?
For right now that's hard to say. I wouldn't count Google out of the competition, they have so many spiders and indexed content they probably have tons and tons of information on different people all around the world. For right now, Facebook is easily in #1 and not slipping anytime soon.
Yes, Facebook seems to be growing, not shrinking. But it's downfall will come sooner or later, nothing lasts forever. It's just a question of WHAT will kill it.
When people will get bored to Facebook then it will start to shrink. It will come after some years. A new website will be born with more interesting stuffs. Or may be it is already there...
Someone would have to buy it out or upgrade it as it is. Users aren't going to cut all their connections just to go to another site. Amiwrong?
Nobody can say yet Facebook does a fantastic job of keeping the site fresh and constantly adding new features and adapting, so I wouldn't expect it to become a memory any time soon Myspace's problem was that it stayed stagnant for so long, while also existing in a state where spammers overran it. By the time the tried to play catch up and add new features, everyone had already jumped ship and didn't care about it anymore.