Have you tried going to the facebook mobile site in your phones browser? After you sign in, scroll to the bottom of the page where there is a link to the specific app version for your curve... Otherwise, I must ask if you have been trying to update the app through the app itself or through appworld... Either way, I would suggest that you delete the app either through DM or options>applications>add-on>facebook then delete then battery pull then try to download the app via one of these options
Posted from my CrackBerry at wapforums.crackberry.com
I had the same problem. I ended up having to go back to Desktop Manager and restore the Facebook app. Once that was done, I got the 'updates available' icon and it automatically updated to the latest version of FB.
EDIT: Restored it from my backup of 4.x before I replaced the OS