Which community to join?

With all these communities being around - Myspace.com for kids and youngsters, openBC and LinkedIn for professionals, Eons.com for elderly people, just to cite a few examples, the question arises how many communities I have to join to appropriately map my social network as I need it (for private and / or business purposes). Is there some "standard" interchange format which contains all the fields I need to represent myself in order to avoid having to join multiple communities manually? Wouldn't it be much better to have some "social integrator" that makes available my data to various community sources?