Probably because a regular email system has no way of "blocking" a user the same way I can block people who are not my friends on those social networking sites from contacting me.

Also, their "mail system" displays ads. More ads, more chance at revenue.

Facebook lets you send email, not receive. I've always hoped they'd make an email system, if only because they've shown themselves to be experts at avoiding clutter.

i guess the (small) issue with facebook is that you don't get a "unique" handle at all, easily changed, I guess.

On the other hand, if a good fraction of users were to get theirname@facebook.com, then the rest had variations on that theme, suddenly sending email to people would be much easier.

I'm happy with my @me, so I guess I won't worry about it too much.