I thought Airbnb sounded silly when I first heard about it, but I've spent the last month in the caribbean staying exclusively at apartments booked there. The UX is awesome, there's a huge discovery value to people with vacation rentals, and it's way cheaper than staying at hotels. Not to mention the money that I save because I have a kitchen to cook in. You can usually find a monthly price only a few hundred dollars more than what you would pay for an unfurnished rental with a lease. I've also been shocked at the explosive growth outside the US. When I checked the listings about six months ago I didn't see much in southern Brazil, but now there are ~500 listings. I don't know about $2B, but I've been very impressed with everything they've done so far.