Mirror mirror on the wall,? Which network is the "social" of Them All?

Last time in class, we learned three different types of social networking sites: The Twitter, Ning and Stumble Upon. Site networking is new to me because I'm really not into internet networking before not because I don't like it..it's just because first of all, I was'nt exposed to it though I heard the facebook and twitter before but I really didn't pay attention to them and second, I didn't get much time to do it. But after we did it in class, I realized some of these networking sites are really cool and "social" because a lot of people are in it . Although I still need to dig in deeper to these sites, I liked the idea of following friends , family and even celebrities in twitter.I also like the Ning because it connects me to my friends, relatives and gives me a chance to create my own social network even with people I don't know but have the same interest as mine.
Stumble Upon is fun when you just wanna explore sites because it serves as a search engine and gives you websites that are related to what you are looking for..but I didn't like it much.