Airbnb stumbled through the first major test of its young life, after the recent ransacking of a San Francisco customer's home kicked up a media frenzy. But as that controversy simmers down, another challenge might already be brewing.

To put it plainly: Some customers in San Francisco and other destination cities who use the online service to temporarily rent out their homes appear to be violating the law. And City Hall and local activists are taking notice.

Airbnb of San Francisco acts as a matchmaker, allowing people to briefly rent out their apartments or rooms to travelers, while charging a service fee. But city rent control laws and planning regulations technically prohibit residents from doing this. The Planning Department defines a property that's leased for less than 30 days as a hotel, which is subject to restrictions, permits, fees and taxes that are largely ignored in these cases.

There's also a California law that prevents unfair competition, which might apply to people who rent rooms on a short-term basis without going through the same process that legitimate hotels and bed and breakfasts do.

"There are a number of reasons it is problematic," said Matt Dorsey, a spokesman for the San Francisco city attorney's office.

Airbnb didn't respond to an inquiry from The Chronicle.

These rules are largely unenforced when people casually rent out rooms to friends or associates, but the growing prominence of services that formalize this practice is attracting increasing scrutiny.

To be clear, Airbnb and similar companies like Wimdu or Roomarama are merely acting as go-betweens in these transactions. But as tourist destinations pass or enforce rules against customers of these services, it raises questions about their future growth.

New York City approved a law that went into effect in May banning short-term apartment rentals. The practice is also outlawed in Paris, where the government stepped up enforcement last year.

Nearby communities including Santa Cruz County, Capitola, Monterey and Carmel have also limited temporary rentals.

In several of these cases, officials cited the impact of the practice on affordable housing.

"By removing a legal gray area and replacing it with a clear definition of permanent occupancy, the law will allow enforcement efforts that help New Yorkers who live in (single room occupancy) units and other types of affordable housing preserve their homes," then-Gov. David Paterson said in a statement, when the law first passed.

Housing activists in San Francisco are airing similar concerns.

Members of the San Francisco Tenants Union say they've found several buildings that evicted tenants in order to convert the property into a tenancy-in-common or condominium, only to instead effectively turn the spaces into permanent hotel rooms. This could not be independently verified.

"It's a lucrative way to make money," said Ted Gullicksen, head of the tenants group, "and it's pretty much become endemic."

"They're evicting tenants and taking rent-controlled units off the market," he added. "And this is all absolutely illegal."

The Bay Citizen recently cited census figures showing some San Francisco neighborhoods now have more second homes available for "seasonal, recreational or occasional use" than there are apartments for rent.

Gullicksen is advocating more stringent enforcement of these rules and says he's been meeting with several sympathetic officials, including Board of Supervisors President David Chiu.

Chiu legislative aide Judson True said the supervisor's office is looking at ways to address the issue, but stressed that it's a complicated problem.

"We do want to make sure that we protect San Francisco's rental stock, without unduly limiting the flexibility of tenants and landlords to allow guests in their property," he said.

A representative of the San Francisco Travel Association said the trade group for the hospitality industry doesn't have a formal position on services like Airbnb, but said the city should ensure that these businesses take measures to protect the safety of travelers.

If they act like hotels, they should also pay the same fees and taxes, so as not to create an unfair advantage, spokeswoman Laurie Armstrong added.

"This is a very new type of business and so I'm hopeful that entities such as Airbnb will find a way to exist and fill their market while also operating in an appropriate business fashion," Armstrong said.

Clumsy PR: It took a while and the process wasn't pretty, but Airbnb on Monday finally owned up to its responsibilities in the case of EJ, the anonymous customer whose San Francisco apartment was robbed and vandalized last month.

In recent days, this column and other publications reported extensively on the incident and similar ones that emerged in the aftermath.

After bungling the immediate PR response, the company issued an unconditional mea culpa and offered up a $50,000 guarantee to protect any customer facing a similar scenario. For our money, that's a far more appropriate solution than the customer insurance option first highlighted, a conveniently revenue-enhancing response that lacked tact in the face of a customer tragedy.

"With regards to EJ, we let her down, and for that we are very sorry," said Brian Cheskyk, chief executive of the startup in the follow-up blog post on Monday. "We should have responded faster, communicated more sensitively, and taken more decisive action to make sure she felt safe and secure."

It remains to be seen whether the response and guarantee will be enough to repair the brand damage and keep the company on its growth trajectory.