Well, facebook was just updated, I suppose it is possible that it is for 4.5 and higher only....but I'm not sure. Where are you downloading it from? Try these steps:
1. On your BB go to Browser/Menu/Options/Browser Configuration and be sure the Browser emulation mode is set to BlackBerry
2. Clear your browser cache
3. Go to http://mobile.blackberry.com and there will be a link there for the 1.6 version of facebook.
If it still doesn't load, post back with the exact error message and we will try to advse you further