Many large American cities have a shortage of affordable housing, but San Francisco's situation is unique. It's a small city, just seven square miles, and its size combined with the legendary explosion of tech start-ups that call the city home, has propelled it and two of its neighboring cities—Oakland and San Jose—to claim the dubious honor of three of the top five most expensive rental housing markets in America.

There's been a lot of handwringing, and some of it has been about Airbnb, which many claim is reducing the supply of long-term affordable housing by making it easier for landlords to rent out apartments on a short-term basis. Recently Airbnb released some data on its users that indicates the critics may have a point, and explained what the company is going to do about it.

Last year (March 2015-March 2016), an average of 17 percent of San Francisco Airbnb hosts' earnings came from short-term, entire home listing stays with hosts managing more than one such listing at time. In a tacit acknowledgement that these hosts contribute to the city's housing crunch (and may be operating illegally), the company will begin expelling their listings starting this week.

But Airbnb, which has fought vigorously against proposed local laws to regulate its business, defended the majority of its hosts who rent out their entire homes.

"Some critics wrongly assume entire home listings have been converted into full-time rentals for tourists and are mostly offered by commercial operators and wrongly classify these as 'unhosted' listings," the company wrote in a blog post. "In fact, many San Franciscans regularly travel for work or pleasure, and countless hosts share their home on Airbnb when they are away."

Ultimately, the company wants to make sure that each San Francisco host has only one listing on its site, a spokesperson told the San Francisco Chronicle. That will help it comply with a city vacation rental law enacted in February 2015, which officials have found difficult to enforce because Airbnb has refused to share information on possible violators, citing privacy concerns.

As Airbnb cleans up its San Francisco operation, it is also facing criticism for other issues, including racism. A Harvard Business School study recently found that those with stereotypically African-American names have a more difficult time renting a room from hosts than those with white-sounding names.