Darren is right on target when he says users don't want to join multiple social networks just for a particular element of their personality. I think I'm pretty much locked in to LinkedIn and Facebook- there's not much else I think I really need- I am not one of those people who thinks twitter is a social network. The smartest brands are creating content that energizes users within their exisitng communities, and creates a lasting association with the product or brand.
A splinter effect of the niche social net phenomenon is the marketer-owned social network, like Saturn's recently launched social network-called ImSaturn. I don't have high hopes- trying to shift a conversation entirely to your site is going to alarm the most enthusiastic elements of your target audience. They'll wonder what they won;t be allowed to do there.
A bigger question in my mind is what the company thinks it is getting out of these initiatives. I still remember when GM killed and recalled is Saturn EV-1, which had about as dedicated a user base as you can imagine- people were evangelists about this revolutionary automobile. GM will never be able to recapture the enthusiasm it had with those users, whose zeal to protect the environment was swept away by a change in corporate strategy.