Nobody will ever be able to legally prove the gmail accounts were created for and by AirBnB. Actually, I don't think those accounts were created by AirBnB guys, but by a third party, for AirBnB use.
Ad extremis, the AirBnB founders might as well plead they never knew what was the exact method this third party will use to generate traffic and users.
Nobody? Get the IP addresses that access those accounts from GMail. You're going to find them all coming from one IP or a small number of American IP addresses. Ask the ISPs who they are. Job done, culprit found.
You think they'll be using anonymous proxies or something? I doubt it.
