i had the same issue the other day (but i point to openDNS) and i dont block it in my opendns portal. i logged into another network that i manage that has openDNS and it worked fine and that network doesnt run pfsense.
after a few minutes if trying, facebook finally loaded, but it was strange. this happens once every couple weeks. i am not on facebook that much to care, i just found it odd that it was not responding.