+1 to all social skill rolls is certainly WELL beyond the power of any stunt. Hell, it's beyond the power of a dozen stunts. It seems to be reasonably costed to me.
They have to recognize and respect the mark for it to work. Seems to easy for the GM or player it is being used against to simply say they aren't impressed. Shouldn't happen, but that loophole leaves lots of room for interpretation.
Since there is that loophole, I see no reason not to allow the +1 to the roll. Marked by Mab could be useful in inciting fear in many related to the Fae courts. Denarians not so much. Though marked by Almighty...may well work in reverse to that situation. It gives Marked by Power a little more teeth in the circumstances it comes in handy, but still isn't as useful as tagging an aspect for +2. Is it a little Munchkintastic and sneaky? Sure, but cleverness should be rewarded. Seems to me the point of trying to manuever, tag aspects, make declarations, and spending fate points is all "jockeying for position" and tipping scales in one's own favor. The system, for lack of better terms, rewards BSing your way into bonuses to conquer the foe.