I think it's a brilliant idea, if it's even true, that airbnb hired more engineers rather than marketing people. It's always better for a startup to deliver a good product, which is in airbnb's instance the actual platform, rather than focusing on promotional marketing like business and greeting cards.
You can get 100 business cards from VistaPrint saying whatever you want with a picture you want. We got business cards thru airbnb a year ago or so ago which were totally useless because they didn't even have our phone numbers. Same goes for greeting cards. But I don't understand how you would send physical cards if you don't know the past guests' home address. Anyway, as both a host and a guest, I wouldn't care for a greeting card from airbnb like I wouldn't care for a text/card from each Uber/Lyft driver who gave me a ride.
However, I would love to see airbnb focusing on perfecting and optimizing their online platform, getting the legal issues in order, helping hosts become more educated and self-sustaining.
Merry Christmas.