Ye they did because they can. Web 2.0 can't be copyrighted, facebook could completely copy everything twitter does except for the name if they wanted to. When it comes down to it in these type of markets, it's whoever has the biggest amount of users which will win.
Twitter's best chance is to try and keep it small and convince users of facebook and myspace to use twitter as well as those, but tbh I don't see why anyone would want to write up what there doing on more than one site. They'll get to a certain size then be bought out.