When I was a teenager (10 years ago) I found out about Facebook through my own mom. Facebook isn't for the youth, the appeal of Facebook was reconnecting with old friends, and family. To a young person, they usually see their friends in school and their family all the time 'cause they get dragged wherever their parents go. The value of Facebook is a storage facility for memories, once you got a large enough body of people somewhere on the internet the rest of the internet follows as a result.

Edit:

I also hear a lot that most people are on FB due to keeping up with "events" as well. Aside from memories / connecting and events it provides no value back to me. I can call / text / email anyone I know, I guess it's just convenience.