I'd cynically like to say political lobbying (and be right, they spend a LOT) but much would be support, development and hosting.

BTW Airbnb charge both the guest and the host, they make a commission from both parties.

International customer service in many languages.

Serious question, have you ever used AirBnB's customer service? It's so spare as to be nonexistent. They make Google look high-touch. The host is providing 95% of the service. AirBnB is only there if the host won't answer messages or there is some colossal problem with the listing. I've had several dozen stays and only invoved AirBnB service on maybe 5 of them. AirBnB should be making money hand over fist since essentially they're providing a pricey listing service.

I'm a super host and can be talking to a real person usually within 5 minutes. I'm in Australia, it might be a time zone thing.

Payroll for a company the size of AirBnB is easily north of a billion a year.