There was a time when people only used Facebook email for informal, personal communications; voices from the past looking to catch up, or casual connections that were anything but urgent.
As folks have begun to get more contacts on Facebook, and use it for more of their daily business communications, the walled-garden approach is no longer functional.
Facebook needs to open up its messaging system to POP and IMAP access, allow forwarding to external accounts, and recognize their role as a legitimate email provider. While I loathe having to have yet another email account to manage, if I could access it via IMAP or forwarding, I could at least manage it. As it stands today, it is really not manageable; in fact the inability for me to archive the data along with my other email communications makes it a liability and something to avoid.
As it is now, Facebook email is actually creating more problems than it solves, and I expect that there will be a backlash against it unless they take ownership of this growing problem. Facebook, are you listening?