The "it" you can't transfer is your rights, not your IP. No, the license you grant them is a non-exclusive RTU. You're free to sell your IP to somebody else, but you cannot abrogate the agreement into which you already entered with FB. I.e.: You cannot post it to FB, then later sell your IP to somebody else on an exclusive basis, because you, yourself, no longer have exclusive rights. You could, however, sell a non-exclusive RTU to somebody else.