To my knowledge, Iranian government doesn't have power to dictate FB's actions outside Iran. So FB could block by the IPs, but blocking everywhere by the nationality makes no sense (at least by the standards of a normal person).
Github once managed to ban accounts of Iranians, even students in the US. Afaik shortly after that GH went to the US government and convinced them that sanctioning every Iranian even outside Iran was technically and politically dubious, which resulted in the sanctions being relaxed in some way.
