Facebook is the best social network. It has the most people which is the main reason to be on a social network. It also has the best design and features. Google + attempted to create a different functionality by forcing people to create "circles", but Facebook already had that capability with groups and recently made it more prominent.
Facebook is better than other social network. Facebook has lots of special feature that other network hasn't.
This is how I see it too and pretty much in that order. I find Facebook more "fun" to use than twitter, despite the new layouts. But of course if you have a highly desirable product then it doesn't really matter where you put it be it Facebook, Twitter, YouTube, LinkedIn, Digg, SU, etc etc. Wherever there are people they will bring targeted traffic. Provided you've got a big fan base and lot of recognition. You're bang in there.
Coming from a marketer for 3 years, I have utilized both networks Facebook and Twitter. Facebook is getting a lot harder to make profits with and twitter has been growing massively throughout the past year. Both are great just find ways to monetize them! Check out some networks that offers help, like ours.
I think facebook is very good social networking site for personal purpose, typepad is also good for blog promotion site.
Other than the obvious, I quite like these too: Yoorl StumbleUpon Linkagogo Clipmarks Mixx It depends what purpose you want them for really!