Appears to be a issue that has to do with google and facebook when they developed ICS. I read something to do with they felt it was some sort of privacy violation or something to pull that info from facebook. I think it was also mentioned that it could be fixed if Facebook were to update their app to meet googles terms or something.
I have read there are work arounds like this http://www.slashgear.com/galaxy-nexus-ics-rom-restores-facebook-contacts-sync-13201934/
Don't know if it works haven't tried it as somehow mine are now linked . I originally linked them manually one at a time but had to do a factory reset but they ended up relinking or something.