Twitter and Facebook share a few characteristics. They are networks of people and companies. Each user has an identity and a feed or wall where he gets some news through the actions of his contacts. With Facebook the user selects his contacts by requesting friendship and accepting requests. With Twitter the user selects his contacts by following. And here the important similarities end.
The way you select your contacts is very different. The result is that users tend to have different contacts on the two networks. On Facebook your contacts are people you have met and perhaps meet with outside Facebook. On Twitter your contacts are people you most probably never met, but you find interesting. Another important difference is the way contacts are made. If your Facebook friend request is ignored, your friendship with that friend outside Facebook may cool down or worse. This does not happen on Twitter, because there's no mandatory reciprocity in following someone. I can follow anyone I like and they usually don't care much, and may not even notice me if they have many followers.
Twitter is open and Facebook is closed. I have to have friends to gain from Facebook. On Twitter this is not necessary. Following someone is a convenience rather than a must.
Much due to the contacts I have on both networks, I find them useful in quite different ways. Facebook gives me access to what my friends are doing or thinking. Twitter gives me access to what complete strangers, including celebrities, are up to.
A skilled twitterer producing many tweets can be great fun. Such a person can indeed wield quite some power: if someone with millions of followers endorses or rejects something, it's going to have noticeable consequences, and the results will show up quickly. I often click on a link that one of my contacts likes.
This is an interesting theme, which I'm sure I'll come back to. But enough for now and I wish You, valued reader, a prosperous 2012.