I definely think they are both great, and each one has advantages over the other. Facebook is better for interacting with friends and family, and has more members and usage Twitter is more simple and is a better business tool. If anyone on here wants to, I have a thread like this one with a Poll, if you wouldn't mind submitting your opinion.
Facebook and twitter both are good. The only thing is that you need to be real active in the usage to get the benefits.
I like both of them and have accounts with them, but if you based it on Page Rank then Facebook is ahead by a single point against Twitter.
I don't think Stumbleupon was ever really a social network, more just like a fun tool to use. I still have it and use it all the time, because it's awesome. But I don't think you can compare it to FB or Twitter
Facebook, I noticed though that some people use Facebook like Twitter. I have a friend that post every couple of hours on Facebook and I told him he is using it like Twitter.