If tours and activities are the next bigthing in online travel, then TripAdvisor is taking note - by adding a third supplier of products in recent weeks in the form of Viator.
Not announced officially by either TripAdvisor or Viator as yet, but links to Viator product pages from a number of attractions have started to appear in recent weeks.
These include some of the City Guide mobile apps and also on attractions associated with city pages on the main TripAdvisor site.
The addition of Viator as a major supplier of tours and activities (and the ability for consumers to book services) follows the first wave in March 2011 when (recently funded) GetYourGuidesigned a deal with TripAdvisor to give users the ability to secure attractions via a "Book Now" button.
This was initially set as a two-year deal for both desktop and mobile services and was understood to be exclusive for the first 12 months. Expedia-hosted tours and activities were later added and now Viator has also joined the fray.
With multiple suppliers on some products, however, TripAdvisor may well have created some kind of algorithm so that it serves the booking links randomly.
NB: TripAdvisor and Viator have not responded as yet for requests for further information.