Are you sure about that? Facebook has an IM system, so it is as bad as IM, and the standard message system is like email, so it's as bad as email, then it's additionally as bad as all the other facebook stuff!
The fact that facebook sees fit to email me to tell me that someone has sent me a message is the thing that makes me want to kill it dead. I know I can turn it off, it's not getting the email that pisses me off, it's just a reminder that email is millions of times better than facebook messages.
Facebook is, more than anything, a fucking mess. I know where all my emails are. I can sort them, I can search them, I can filter them, I can put them in folders, I can set up rules to put them folders. Email is an actual functioning useful technology. I don't know what facebook is.