10-16 year olds also live with their parents and often "hate" them. Now that more adults are there, it's not as natural to get on Facebook as a teenager/pre-teen.

Will they join later when they move out and seek ways to stay connected with family? Who knows - but I can definitely see why they're not there yet. I'm older, but the only reason I joined Facebook was to have a way to keep in touch with family once I moved 2000 miles away to California.