This is a trend that has happened countless times in the past, and will continue happening for years to come.
Teens go somewhere. Parents and grandparents follow. Teens leave. Parents and grandparents follow.
I can name 2 real-world examples: Myrtle Beach, South Carolina, and Daytona Beach, Florida (I've lived in both, so I speak from experience). 20 years ago, both cities were THE place to be for teens. Now, they're retirement communities that roll up the sidewalks at 8:30pm.
Friendster, MySpace, they've all had the same fate, and there's no reason to think that Facebook has greater longevity.