I think it is not about who is the best or what is the most promising Social Networks. It's all about monetizing. And to be able to monetize your site, you can try to use all the available and famous social networks such as facebook, twitter, digg, stumble and many many more.
I am using twitter for marketing and i am getting lots of success using it and never tried for facebook yet
Facebook has given me the best results so far as well. i try to meddle with them all, because you never know what features will come out in the future