First, I'll recommend this Mimi Ito interview from PBS. It's just a few minutes long. In it she points out how kids don't generally like online social spaces where their parents are.
Watch the full episode. See more Digital Media - New Learners Of The 21st Century.
One element that stems from this observation is about the future of the Facebook userbase and Facebook itself. As its users get older and more of them become parents, will their children want to use that place where their parents hang out online? Probably not.
I could be dramatic and make statements about Facebook's not-so imminent demise, but that assumes all kids act the same, or that kids won't carve their own spaces out of the larger Facebook "Zuckerberg says we should share everything" space. It also assumes stagnation on the design side of Facebook, and they've been a moving target for several years, although usually in the pro-commercial side (share!) and not the pro-user side (privacy if you want).
I'm also not going to predict that in about 10 years we'll see all the current pre-schoolers decide on some online space that isn't Facebook. Given technological change, and the ease of making a social networking website (relatively speaking), it is difficult to predict how we will conceive of, talk about (academic: framing, discourse...), and label future online spaces. MUDs were in some ways early social software, but we didn't call them that (although people did point out the social side at the time).
But, to use market-oriented terms, there is clearly an opening for such a space. Could we see a resurgent MySpace? What happens to kids as they grow out of those "kid spaces" that seem odd to me since I'm an adult (and I don't follow them), what are they, Penguins?, Disney?, Habbo Hotel? Hobo Hotel? I don't know, they aren't marketed to me (as neither a child nor as a parent).
But, if kids want their own spaces, will Facebook be able to provide a space where kids feel it's their own? Given the number of adults on Facebook currently, and that teens on Facebook currently will grow older and many will have their own children, where will those children go when they are online? Will we even call it online at that point? (Probably, given that word has been around for a while and transitioned to today from modems, walled-garden online service providers like AOL used to be, and other dial-up services like BBSes and mainframes back in the day.)
Facebook's current userbase is a factor in its future userbase, and there are reasons to believe some of the current userbase will be a drawback for the future (youth) userbase. This problem is not, I don't think, easily overcome.
