Two points: (1) this is U.S. traffic only --- more than half of our traffic is international (2) the slump corresponds with the seasonality of U.S. travelers; compare with vrbo.com for a similar example.
It's about travelling and therefore going to be highly seasonal. You need to see data from a couple of years to really see a trend.
Yeah, people were booking for the holidays ahead of time during the summer. The holidays are peak time for Airbnb and other travel companies so I'm not sure what the original poster thinks this shows exactly. Certainly people weren't traveling less in December!
I can't see the graphs in the article (on the iPad in a cafe), but I wonder how closely the traffic slump correlates with the bad press they got late last year over the girl who's apartment was trashed?