Airbnb recently filed suit against San Francisco over a new rule governing short-term rentals, which the city's Board of Supervisors approved last month. An Airbnb-supported law adopted earlier this year requires short-term rental hosts to register with the city, but it's estimated that only about 20 percent of them have done so. The new rule requires short-term rental listing services like Airbnb to enforce the law by ensuring that hosts advertising on their websites have registered with the city before posting ads online.