Obviously some people who think Facebook is silly will always think it's silly. But for current Facebook users, I think it will only get more useful to them as they get older.

Up to a certain point in life, getting older usually means more responsibilities; jobs, kids, etc. That's less time to maintain and cultivate face to face social relationships.

I think you'll find Facebook's population is only getting older as people with limited time go online to create and maintain those relationships.

update: I bet every popular web service eventually will have enterprise-oriented version, like Yammer is to Twitter.