email sucks. people never check it, and what if you don't know someone's email address?
on facebook, you can just look them up by their name and send them a message. it puts everything in one place. much easier.
I check my email pretty much every hour. I send about 30 emails a day. Facebook mails me when someone sends me a mesage via facebook. These emails get redirected to a folder of their own.
At the moment, I have 200 facebook message notifications in there. None of them have been checked. You want me? Email me, or text me or send me a letter. Don't do it thru facebook. I'm not interested in what you had for breakfast.
That said, there's an interesting divide here between these who download all their email to their own computer, and those who depend entirely on webmail and other online-only communications. For me, Mail.app searches through 5 years of email instantly, in the blink of an eye. That's too precious to give up.