TwHistory let students reenact historical events in a pretty easy and neat way. You can send tweets from historical figures, but you can do it through the site without actually having to sing-up for Twitter (which is problematic for most schools).
You can read more about the site at the MindShift blog.
It’s certainly a very viable and engaging option, and I’m adding it to The Best Tools For Creating Fake “Stuff” For Learning.
I think students are more familiar with Facebook, and would rather use the fake Facebook page sites on the list, but I might be wrong….