So interesting (or maybe obvious?) which ones have really succeeded and which ones rapidly fell into obscurity. Perhaps there is a simple, universal reason why some fail and some succeed?
hughbot answered:
some are hard to use and ugly and hard to convince your friends to join.
forbetaorworse answered:
(i think the launch of facebook for college preceeded the high school launch) intrusive ads old people and spambots kill social networks IMO