Airbnb is getting bigger. As is the number of San Francisco housing units set aside for renting out via the website, according to reports.

As many as 7,500 San Francisco housing units are kept off of the rental market and are instead set aside for users of Airbnb and services like VRBO.com, KALW reported.

Activists with the San Francisco Tenants Union identified 1937 Mason Street, a three-unit building, as apartment housing set aside entirely for vacation rentals, the radio station reported. To make matters worse, the former renters there were ousted with the Ellis Act, activists reported.

No "official" estimates on how many units are rented out to tourists exist, but the Tenants Union say they've been watching the market for five years and have watched it grow.

Airbnb is worth billions of dollars, according to recent market estimates, and the number of listings on the site have grown exponentially since 2012, KALW reported.