Arguably one reason those two businesses were successful in areas with entrenched players and business practices was because they handle the money. If AirBnB was asking either travelers or hosts to pay $X to be on a recommendation site, probably very few people would. There's always a cheaper competitor when you're selling information. Because you book through AirBnB, for a service which is relatively expensive, they can skim off quite a lot of money in an opaque way.