A Morgan Stanley report this week found that Airbnb's growth is slowing in Europe and the U.S., and Expedia and Booking Holdings are making gains. Although Airbnb disputes the numbers, this race was almost inevitably going to tighten. These are very early days for alternative accommodations, and leading players will dip and fade over the years. Anyone remember when Travelocity was the world's leading online travel agency? Anyone?