Plenty of other exceptions: a large % of eBay users probably don't use the site every other day.

Selling on eBay or renting a property are highly intentional decisions: trip planning is something that will happen. The trip itself is intentional, but the exact itinerary will vary, and the lack thereof won't prevent the trip from occurring.

I don't agree that 20% retention statistic is a good way to view it at all. There are tons of exceptions to that rule - Amazon, Kayak, Turbotax, or even TripAdvisor. I often view that as entering one's normal "workflow" through whatever it is they are trying to do - it's a rarity, and there is a good chance that a fringe app wouldn't make it.

There are two unique userbases for AirBnB - renters and renters. Renters may not have a 20% daily retention but I would bet that rentees are much closer to 20%.

On top of that, AirBnB is a marketplace. I would imagine that marketplaces are affected more by supply and demand ownership than daily retention.

Don't forget that there's a distribution around the mean. You'll probably find plenty of outliers. It'd be interesting to see the graph MixPanel's guy is basing his estimate off of.

Let's call it a rule of thumb. There are always exceptions in startups.