To classify a company as a "startup" I'd go with either Steve Blank's definition (An organization in search of a repeatable and scalable business model [0]) or Jacobellis v. State of Ohio (I know it when I see it [1])

Sure, AirBnB certainly appears to have hit the scaling part. But I bet they're still trying to find new models and new markets. Hilton, Marriott, Starwood and others dominate the corporate travel market. Dropbox appears to be moving towards photo sharing on the consumer side and config/settings sharing on the tech side.

Startup, not startup. Probably irrelevant. I know that there's value in having companies who think like a startup as vendors. Procurement departments aren't structured to be allowed to think that way and I think that's bad for the companies they serve.

[0] http://steveblank.com/2010/01/25/whats-a-startup-first-princ...

[1] https://casetext.com/case/jacobellis-v-state-of-ohio/paragra...