'New media expert' is a pretty lame title, but he definitely has something to add. He is an internationally recognized celebrity, and Airbnb is a company crossing the chasm and trying to pull in the laggards.
Convincing people to use Airbnb is going to take more than ranking high in Google and having the proper precautions in place. Ashton brings insight to the rest of the world.
If anything, I'd say he's an 'old media expert'. Or, maybe a branding and publicity expert.
I've got lots of respect for the AirBnB team. They've done a great job building their business and if this helps then good for them.