I am a big fan of Twitter. Also use Linked In and Facebook (but not really for business). Have a Friend Feed account but have not had time to figure out the benefits yet. Plus of course, I use and enjoy Ning.
I think I am just getting to the point of really understanding the potential of social media sites.
My first love was twitter because it really gave me insight into people. Do they just market? Do they live near me? Then came Facebook. I like Facebook because you can have more in depth conversations with others. I enjoy the benefits of having a Facebook group and the interaction and exposure that it offers me.
I am taking some of those relationships even further through Ning and the various groups I belong to where we can have private conversations with the boundaries of Ning, feeling like a safe wall.
Still new to Social Media, but am most comfortable right now with Twitter and Facebook. Learning how to blog, also using Digg for bookmarking interesting content from many different authors.