For anyone not aware (or confused by Silicon Valley tradition on constantly rewriting origin stories), TripAdvisor started in the dot com bubble as a B2B site. They clearly stated their business model was to provide reviews to other sites. Only that after the bubble burst, TripAdvisor had none left. They then basically invented modern SEO: long descriptive URLs, pages pulled out of a db with keyword variations in mind, constantly refreshing content with user submissions, and, the killer, buying links to influence Google’s naive ranking methods (which quickly evolved this to buying entire sites for their SEO potential). The billion dollar public valuation is entirely the result of a decade long SEO play.
(Source: was in online travel related business at the time. Was the first to buy links form Matt at SeatGuru, later acquired by TripAdvisor)