These days I have been using a twitter quite a lot. I've used facebook, CyWorld(A Korean social networking site), and Mixi (A Japanese social networking site), but none of them solve the problems I have: Privacy and Simplicity. Once being a friend, somehow I know where friends have been, they know what's going on me by looking at pictures, walls, etc. If I block them or apply restricted permission to them, they'd notice it because they know they cannot access some parts that they used to access before. Also they are not simple to use. If I go to Facebook, my profile looks very disorganized and very complicated in a look.
I don't know about privacy on twitter; but I think I like twitter's simplicity. Moreover, what I really like is the fact that I don't need to spend my time to do something on twitter. Whenever I want to write something on twitter, I can just open the twitter by "toggling" on the computer and write it. Or I can tweet anywhere using my phone. I don't need to spend my time in front of computer to "organize" my wall and my contacts. I think that's the strength of it. I don't need to open a browser to do something complicated.