I'm a massive fanboy so I may be biased in saying this, but I can't recall a blatant Messi dive. I also think that he tries his hardest to stay on his feet when he's tackled, which is why you always see those mazy dribbles where he's bouncing off 3 or 4 players. If he was Ronaldo, he would have been on the ground after the first tackle, foul or not.
Some of the other players in his team are admittedly quite quick to fall to the ground - Busquets, Bojan and Pedro were egregiously going down at the slightest of touches - but like I said, this doesn't apply to Messi.