So you still have the actual Facebook app? If not, load from the market.
If so, I believe the HTC (system app) would be located in /system/app/HTCFacebook.apk
Perhaps try to find/launch it from there and if it doesn't exist, I attached it to this post as an .apk which you should be able to load so long as you have enabled "unknown sources" at Settings > Applications.
Another caveat, I am running leaked 2.2, not sure if my .apk is compatible with your setup or not. Do a nand backup before attempting.