Any new social network has the same problem that other social networks have - how do you get fans when the only value proposition you have is interacting with other people? It's a catch-22. You're definitely right about utilizing addictive game elements and social elements. Farmville, for example, isn't even a game. But people spread links for Farmville because their UI says stuff like "Robert needs your help building a barn" or other stuff like that to make you feel guilty. Other ways of jumpstarting your site include advertising on Facebook or even growing your Facebook page on GetMorePopular and other techniques. Facebook is clearly the best place to grow your site because when people post about something all of their friends see it.
the mentionmap is sweet! I don't think I've seen that before, I've seen screenshots I think, but have never seen the name of the site! Thanks