Rafe Needleman comments in this interesting Release 1.0 article about why social software sites are 'misguided' - he really means transitory. As he says, "the more powerful a community site, the easier it is for the alpha users on it to get people to switch over to another site".
Churning to a site usually makes churning away just as easy. Admittedly, "The biggest community sites are populated by young people, an audience that is constantly renewing itself", so fashion moves on, so does the clientelle: Friendster to Facebook to MySpace to ???.
In conclusion, segment and don't be all things to many people as possible - do one thing well and hope that you can become recognised for sticking to the knitting.