Dan Schawbel put together a compelling piece on signing up for social networks to claim your name or suffer. Wearing my real estate lawyer hat, it is all about three things: location, location, location.
Joining social networks does not cost you anything other than a few minutes to register and add your information. You may find it interesting. Even if you do not find it interesting today, you may find it interesting in the near future.
Claim you name on these social network sites. Even if you do not use them actively, you can generally point a lot of information at them from other collections.
Even if you have a fairly unique name like my name, there are still others out there with the same name. When I first became Doug 2.0 and started my online presence, "Doug Cornelius" was mostly about the Yuba College basketball coach. Since then, the top 20 search results in Google for Doug Cornelius all point to me. (At least as of this morning for my search).