Haven't the slightest idea why, but today it worked in the Pictures & Videos app--I was able to select FB. It interfaced with Picture Mail just fine. Now to check the actual site and see if my sample pic made it there...yep. Adds the tag "Sprint Picture Mail" - hope that's customizable.
I tried
mobile@facebook.com through my phone's gmail, and it rejected it. (I did sign up for mobile confirmation at FB, yes). Maybe it has to go through Picture Mail anyway.