I hate to admit that I am on both twitter and facebook and I read them religiously and post to them quite a bit. However, to me, they have very different purposes. I know this is different for everyone, and it should be.
I use facebook primarily for friends in real life or people I went to school with or whatever. I use twitter for people I have not met, but based on their interests or mine, would maybe like if I were to ever meet them or even lived near them. The good thing about this is that I have varied interests and while there are not many people with the same combination of interests as I have, there are a lot of "specialists" out there with one of my interests and they will talk about it in great detail!
This may not work for everyone, but I do think it is important, at least for me, to keep these two areas separate, and this method of doing that works quite well for me.
I think of both groups as my friends, which I know might be pathetic, but it works out well that I know that I might run into one group of people and have them recognize me.
Does it change what I write? Not yet, but it might. I know that a lot of people might put stuff up on a site where they don't know people that might sound more critical then they are in person, but that just isn't me.
Do you twitter or facebook?