If facebook can get one, or both, of these from its 350 million users it has a significant chance to stick around. You also have to consider teen markets shift much faster than adult markets. I know in my high-school a single thing could spread throughout the whole school, but when a new grade entered the viral spread had disappeared and they remained 'uninfected' by it. So any trend automatically lost 1/5 of its teen market if it didn't capture the new grade.
This happens less so with more mature crowds, who tend to be more loyal. You don't see people buying from the same car manufacturer time after time because it's an 'in thing', it's because they're loyal, which is in essence reliance and trust.
Facebook-today is, for my uses, much worse than Facebook-of-years-past.
In the past it was simplistic, elegant, and did one job and did it well. Now it is trying to be everything and is bloated, slow, and spammy. The signal-to-noise ratio is terrible and I can't find a way to turn off "Mafia Wars" spam.
On the notifications that display, hover over the top-right corner and an X should appear. It prompts to hide all notifications from the application.
Thankfully notifications are one thing Facebook is removing. Perhaps the signal-to-noise ratio will improve when they make the spamming more difficult.
Everything goes in waves though, it lies in human nature to grow bored with sameness. Ten years ago there existed a community product in my country that had managed to capture 90% of all high-school students, and they seemed unstoppable. Now, the product is almost forgotten, they lost to other products, similar products, but people had just gotten tired of it and wanted something new, something cool, something different.
But ten years from now Facebook will probably be gone. It will be very interesting to see what replaces it though.