I think you have the best hand often enough here to call (1 in 12-14, depending upon number of callers behind you). Although Instabettor probably has you beat, I think there is a substantial enough chance (10-20% IMO) that he is bluffing at it by representing a straight or trips (or quads or 2-pair).

I admit that the players behind you make me less confident that this is the right decision (I'm folding to a raise behind me), but I think this is one of those situations where you don't fold on the river: that is, a decent hand in a big pot.