Looks like Microsoft hasn’t forgotten about Windows Mobile and released an update for the Facebook app as of 11/24 that includes features that the Windows Phone version doesn’t have. Here’s the info:
New from Windows Mobile: a Facebook application for your phone! Download the new Facebook application for Windows Mobile and: Send messages to any of the people in your Friends list. Take pictures and videos on your phone, then upload them right to Facebook. Send messages or call people in your Friends list. Manage your profile and post anytime, anywhere. Keep up with the latest news and posts with Facebook on your phone. Now your status updates can be up-to-the moment accounts of what you’re doing. Photos and videos are about as close to live-action as you can get. Show your friends what you’re up to, while you’re out and about. Connecting and sharing on Facebook just got a lot livelier!
Send messages to any of the people in your Friends list.
Take pictures and videos on your phone, then upload them right to Facebook
Call people directly from your Friends list
Manage your profile and post anytime, anywhere
If you’re still using WM6.5 may as well go for this update.