I ran across this interesting article today, though slightly outdated, related to Facebook email -
5 Connected Marketing Tactics to Prepare for Facebook Messages.
As new email providers emerge (ISP based, non-ISP based, social network based...), marketers would be better suited to think about their subscribers like good ol' mailed magazine subscription subscribers.
1.) Take a look at your subscriber list and see how many domain names are "facebook.com". My guess is probably not many so far but what about your customer segment? Is it likely many of your customers will soon wish to receive their marketing messages via Facebook instead of traditional email channels? Will they look at their Facebook email more often than their standard email. If the answer is yes, it would be a good idea to provide your opt-ed in customers w/ an option to switch their email address very easily. Send them an email if it looks their engagement has been low lately to make the switch. Never hurts to provide some kind of incentive as well. Remember those mailers you use to get in the mail to renew your subscription and receive 90% off the cover price?
2.) Most retailers have a Facebook fan page but haven't really been keeping it up to date. And if you don't have one, now is the time since email sent to a user's Facebook email may never make it there if you aren't in the users network due to privacy settings.
3.) Make sure the emails you send with promotions are updated on your Facebook page and always provide the option to "Like" your brand. And don't forget to send customers (existing and new) back to your website to either update their email address or "refresh" their email address to receive a free gift.
4.) Remember that whether a user adds you to their network or not, is a reflection of who that user is. They won't accept everyone just like you wouldn't sign up for every magazine subscription that comes along. Consider your Facebook fans to be your most valuable brand advocates yet! This means pay attention to what they are saying about you.