Bad mouthing a market you're about to buy into isn't so dumb. If he were super enthusiastic about Facebook in the first discussion, Microsoft might have had to

Bad mouthing a market you're about to buy into isn't so dumb. If he were super enthusiastic about Facebook in the first discussion, Microsoft might have had to pay more to get the deal. "It's just a fad" == "I can walk away from this deal" -- one of the best negotiating positions possible.