I've been laboring under the assumption (informed only by common sense, no inside information) that all of the social software bubblers are planning to announce that their version of Friendster is a killer app because a mediated whitelist for messaging based on your circles of acquaintance is spam-proof. It solves the email problem!
Except if you consider a proprietary, closed, privately-owned messaging platform a problem. Clearly, millions of instant messaging users, myself included, don't necessarily see it as a dealbreaker. But I rarely check the messages people send me through Friendster or Tribe, and I never check the bulletin board or group messages addressed through those sites, so I'd say that's a tough sell. But then again, I'm an old-fashioned stickler for things like having a product or service that people actually pay for.