The only substantial difference between facebook and traditional email is the restriction on who can send you mail. If all of the people you interact with are

The only substantial difference between facebook and traditional email is the restriction on who can send you mail. If all of the people you interact with are all on facebook already and use it often, it doesn't make sense to use anything else. The fact that you can't get email directly forwarded to you is very annoying, however it is a bit more secure. (edit there are other differences of course, but they are "non-substantial." The restriction of in-network sending is the only thing that makes it not "email" already as far as this discussion is concerned. If you could message myname@facebook.com then it would be email.)