Facebook just hit us up with some very good news: it knows the email system has screwed up contact lists around the internet, it knows what the problem is, and most importantly, it's fixing the mess tomorrow.
According to the Facebook engineer we spoke with, the root of the problem is a "longstanding" bug with the way your mobile devices talk to Facebook when syncing contacts. The system is designed to pull down your "most recent" email address added to your Timeline, and then add this address—not overwrite anything—to a given friend in your address book. This makes sense, as the email account you've most recently added is in all likelihood the one you care most about.
But then Facebook started auto-adding @facebook.com email addresses to our profiles without asking, meaning our "most recent" email address was one we both didn't know about and didn't want. And then it started deleting the existing one whenever it linked up with your address book. These two problems hitting at the same time meant your friends' contact information was both updated with junk info, and replacing what you actually wanted.
Facebook says the bug "only affects a small number of users," and that these unlucky folks are "mostly older Android devices"—but either way, Facebook is cleaning up its act starting tomorrow, meaning all it should take is a fresh contact sync to put your contacts back to the way you wanted them. Whatever your friends have designated as their primary email address—assuming it's accessible to you via privacy settings—will be the one downloaded to your phone. Period. Phew. Finally.
Just make sure you un-fucked your primary email address to begin with.
Update: A Facebook spokesperson sent over the company's official word on both disappearing messages and syncing bugs:
Facebook Messages is designed to give people control over who they receive messages from. In account settings, people can specify whether they want to receive messages from Friends, Friend of Friends, or Everyone.
If someone sends you an email to your @facebook.com email address and it's from an address associated with a Facebook friend or friend of friend's accounts, it will go into the inbox. If it's from an address not associated with a friend or friend of friend's Facebook account, it will go into your other folder.
However, if you've specified in privacy settings that you only want to receive messages from friends or friend of friends, then the message will bounce.
We've noticed that in a very limited number of cases, the bounce e-mail back to the original sender may not be delivered because it may get intercepted by spam filters.
We are working to make sure that e-mail senders consistently receive bounce messages.
Contact synchronization on devices is performed through an API. For most devices, we've verified that the API is working correctly and pulling the primary email address associated with the users' Facebook account. However, for people on certain devices, a bug meant that the device was pulling the last email address added to the account rather than the primary email address, resulting in @facebook.com addresses being pulled. We are in the process of fixing this issue and it will be resolved soon. After that, those specific devices should pull the correct addresses.