How much can we trust a Google-driven 'open' social networking platform? They did purchase doubleclick after all. While these social networks are interesting projects, I don't think it's a good idea to allow a single company like Google to run the underlying network and maintain the user information. Certainly such a "platform" is not stable in the long term for building startups on if for no other reason than the social graph owner(s) can shut you and your applications out on a whim.