Critics of Airbnb have long claimed that its vacation rentals are removing units from the housing market, but data to back up that claim have been hard to come by. Now a big, five-part report from the San Francisco Chronicle confirms that although most Airbnb listings are rented out to travelers just occasionally, there are at least 350 properties that appear to be used as full-time vacation rentals. That is a small portion of the 5,459 total listings that the Chronicle found in its deep-dive of Airbnb data, and a fraction of the roughly 925 to 1,960 units a recent report commissioned by the Board of Supervisors found. Still, that 350 is likely to have some impact in a city starved for housing—and is tantamount to losing nearly 10 percent of the 3,514 units that were built citywide in 2014.

The Chronicle analyzed data harvested from Airbnb on May 19, 2014, and the same date in 2015 to assess year-over-year changes. The number of all Airbnb listings across San Francisco increased by 13.8 percent over the past year, although there has been significant churn in available listings, with many leaving the site only to quickly be replaced by new listings.

The Mission was the most popular neighborhood for Airbnb rentals, with 789 total listings, up from 681 in 2014. Properties in the Mission were reviewed 6,231 times over the past year, indicating that Airbnb users have been active there. Other neighborhoods with lots of Airbnb listings include SoMa with 388, the Western Addition/NoPa with 369, Bernal Heights with 264, and the Richmond with 262. However, Pacific Heights and Fisherman's Wharf were the most expensive places to stay, with average prices of $288 and $287, respectively. The average price citywide came in at $202, up $19 from the same time last year.

That Airbnb's rise has coincided with San Francisco's growing housing shortage has fueled intense criticism of the site and its model. The city attempted to regulate Airbnb earlier this year when it passed legislation limiting short-term rentals of entire homes to 90 days each year per property (hosted stays, where the Airbnb operator is present, are currently unlimited). But the city has struggled to enforce the legislation. As the Chronicle notes, Airbnb could remove listings after they reach the 90-day legal limit, but the company says that violators would just move to other platforms like Craigslist.

Airbnb also keeps mum on what properties are rented out and how often they are used as short-term rentals. The Chronicle made conservative estimates about property usage by calculating how many times each property had been reviewed over the past year, recognizing that only a portion of travelers will review a property after staying (Airbnb says that about two-thirds of guests leave reviews).

Airbnb claims that the majority of its listings are used as vacation rentals just part-time, and the Chronicle's research showed that 64 percent of San Francisco listings received 10 or fewer reviews during the past 12 months, indicating fairly low usage. Among entire homes rented out on Airbnb, the Chronicle found 352 that garnered 26 or more reviews in 52 weeks, indicating that guests were checking in at least every other week, a frequency that would prevent the unit from being rented out on the long-term market. And there are 104 properties that received 51 or more reviews in just the past year, indicating very heavy usage.

While most Airbnb hosts advertised just a single property, 205 hosts were responsible for three or more listings, suggesting the presence of professional Airbnb property managers that range from hacker hostels to Airbnb entrepreneurs. Those "super hosts" are responsible for 18.2 percent of Airbnb's listings. According to Airbnb, only 10 percent of Airbnb hosts are renting out space where they do not personally live.

The debate over Airbnb, its effects, and how to regulate it and other short-term rental sites is far from over. Mayor Ed Lee ordered the creation of a new vacation rental regulation enforcement office last week, and a measure to regulate vacation listings more rigidly is headed for November's ballot. Having the stats to back up the issue is undoubtedly helpful, although there are no clear answers to the regulatory questions that surround this still-new but growing world of short-term rentals.

· The Airbnb Effect [San Francisco Chronicle]

· More Units in the Mission Are Rented on Airbnb Than Were Added in All of 2014 [Curbed SF]

· Airbnb Loopholes [Curbed SF]

· It's On: Airbnb Regulation Set to Hit San Francisco's Ballot This November [TechCrunch]