@jake Does a link where you can see more details about a hotel, images, availability, and even book it count as advertising? It's not different than a link to a Facebook page, except if you book something on Kayak, OM gets a bit of your money. Consider it as an experimental form of donation.
@organicmaps @jake Yes, referral links are a form of advertising. The difference between that and a Facebook link is you don't get kickbacks from Facebook links to incentivize them over competing services.