I think cool and hip is 16-25 year olds, not 10 year olds.
The reason it really doesn't hold at all, in general, is because many websites (and particularly their contents, not necessarily the sites themselves) cater to different interests, and it's really not a surprise that people in different age groups have different interests. It's also certainly not surprising that Reddit caters to the interests of younger people more than Facebook does.
So if one wanted to create something to rival facebook, create a place that suits the needs of the younger generation better than facebook does, the rest will eventually follow.
Even reddit seems to have a healthy following amongst "older" people.