I experienced the same problem once but I dont know why it happened. But the solution was deleting the last few users of the sql table and after that everything worked fine.
It as well may be, that this is a facebook problem, as from time to time their servers are very slow and do not deliver all information requiered by Your site at time.
There was an older post related to that here
http://www.question2answer.org/qa/3366/fb-avatar-does-not-show-up
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