As someone that pretends to be on top of the whole "Internet" thing, I try to maintain accounts on all of the services, and keep abreast of all the social networks, and all the various communications methods on the Internet. This of course is a silly and foolish goal, but, none the less...
facebook is by far the leader in the social game (some would claim winner), that being said, I really have to give kudo's to Plaxo. Although their membership is really limited to the nerds right now (that may never change), their integration with anything and everything is very admirable. As an old Apple fan (from pre-Mac, pre-windows, pre-IBM and pre-PC stantpoint) I know that a lot of superior technologies don't win. The Apple ][ came more open than anything that has ever been in my mind. The resource manual came with the bios commented source code and fold out schematic of the motherboard. I don't think Plaxo will ever really "be" anything, but, I think they are worth taking a look at, if only for academic purposes.