Ugh, to be honest I get tired whenever I see the "Why did company XYZ need so many people, they're just a website!"-type comments. While it is definitely possible AirBnB was bloated, it's not hard for me to imagine at all what all these people did.

AirBnB is a relatively high-touch business, so I imagine a huge number of those people were in customer support/customer relations, both for travelers and for property owners. AirBnB also operates in a huge number of countries, and each of those countries need (a) marketers, (b) people with regulatory knowledge (often at a level much more granular than the country level - and to head off any 'but AirBnB ignores the regulations!' comments, while that may be true, I guarantee they still have people that know what they are), (c) again, customer service people knowledgeable with the local language and customs.