Well, this is what MAY be happening on Facebook. Statistics from this article: http://www.readwriteweb.com/archives/facebooks_own_estimates_show_youth_flight_from_sit.php indicate that this may be happening. It makes sense, as what self-respecting teenager would like their parents or grandparents on their "friends" list.

Interesting stuff (I'm still not on Facebook, nor do I anticipate joining anytime soon).