hm, I had one contact (whom I'd had in both google and facebook) who'd had a facebook picture, but then he didn't for a bunch of hours (had the grayed out picture frame, not the picture-less cartoony gray person), and then the pic reappeared (it was the same pic). I had another contact (again, both google and facebook) for whom I had a facebook pic, then that disappeared into that grayed out picture frame for that person.
Just over two weeks later, that second contact's facebook pic still hadn't shown up. And he is someone who does update his FB status pretty frequently, and he'd had the same profile pic the whole time.
Then...I exchanged for a new Pre, and my facebook account was there from the Palm profile backup...but it wasn't logging in or sync-ing right, or something. When I went into the account, I'd give it my password, and it would start syncing, and the number of facebook contacts would go up slowly, then it would come back and say that the password or username was wrong. After several tries with that, I rebooted and decided to remove the account. It took several tries and maybe another reboot before that actually succeeded.
Then I finally added the facebook account back in...and it seems like it's loaded all my FB contacts.
Most relevantly for this thread...I haven't yet come across any FB contacts missing their photos. That one friend whose pic had been missing now has his pic in there, and I try to keep my eyes out for missing pics.
So...maybe it takes just completely removing the FB account and putting it in again? I know that's been suggested...but that's definitely one of the possible helps in what I've had to go through (of course, there've been a lot of confounding variables).