By becoming the platform for all social media
If it takes off like Google wants it to, with third-party developers creating a million apps, Wave could be used for, well, practically anything. Everything that people use Facebook, Flickr, YouTube, MySpace, personal websites and Meetup for, Wave could potentially do, except that everything would work together seamlessly and consistently.
Personally I think it's highly unlikely that Wave will ever get that powerful. I just think it's too hard to get that kind of consistent, seamless integration between different apps, and harder still to get people to detach themselves from established online communities where they've invested huge amounts of time. But I think it's at least a remote possibility, and certainly Google must be dreaming of that.