It would seem that they know each other:<p><a href="http://blog.hipmunk.com/post/6493889280/your-guide-to-a-private-island-vacation-hipmunk" rel="nofollow">http://blog.hipmunk.com/post/6493889280/your-guide-to-a-priv...</a> <a href="http://blog.airbnb.com/your-own-private-island-from-airbnb-hipmunk-a" rel="nofollow">http://blog.airbnb.com/your-own-private-island-from-airbnb-h...</a>
Same way other (orbitz, kayak etc) do it. There is very little money in airline ticket sale cuts. A much bigger market and ultimately how these companies make a majority of their money is thru hotels.