As TripAdvisor ramps up hotel bookings on its own sites and apps, the Priceline Group and Expedia have been playing very hard to get but Priceline’s Booking.com has now started participating in TripAdvisor Instant Booking, and is the first major online travel agency to do so.

Booking.com’s participation — and sister brands Priceline.com and Agoda will likewise join in — has big implications. TripAdvisor views this first strategic partnership with a global online travel agency for its Book on TripAdvisor solution as “a major milestone.”

At mid-day Tuesday, TripAdvisor’s stock price had jumped 23 percent, Expedia’s had fallen nearly 4 percent and Priceline Group’s stock price fell 1.86 percent.

There had been a lot of speculation that Booking.com and Expedia — which together accounted for 46 percent of TripAdvisor’s total revenue in 2014 — would reduce their marketing spend with TripAdvisor while the site that previously was known primarily for its user-generated hotel reviews evolves into a quasi-online travel agency and hotel booking site.

But Booking.com is heightening its participation with TripAdvisor now — and that’s a testament to TripAdvisor’s effectiveness as a marketing vehicle. Expedia, which owns a hotel metasearch site of its own, albeit one of a more diminutive nature in Trivago, could take a similar or different path, including throwing its lot in with the emerging, but much smaller Book on Google. That remains to be seen.

The TripAdvisor-Priceline Group agreement on TripAdvisor Instant Booking almost immediately transforms this Book on TripAdvisor feature from a marketing channel used primarily by major hotel chains such as Marriott, Accor, Best Western, Mandarin Oriental, Choice Hotels, Carlson Rezidor Hotels, and Preferred Hotels & Resorts, along with a smattering of small online travel agencies, including Getaroom.com and TripAdvisor’s own Tingo, into a platform where at least one big online travel agency will participate, leveling the playing field.

With participation initially by Booking.com, the three sister companies, namely Booking.com, Priceline.com and Agoda, will contribute hotel inventory to TripAdvisor Instant Booking — and not the apartments and homestays that both Booking.com and TripAdvisor are aggressively on-boarding to their respective sites and apps.

Booking.com’s agreement to participate in TripAdvisor Instant Booking likely came with branding and economic concessions by TripAdvisor, but the nature of Booking.com’s branding in TripAdvisor Instant Booking couldn’t be ascertained as of this writing because it hadn’t yet gone live but was expected to imminently. In the previously stated view of Priceline Group CEO Darren Huston, the lack of branding for partners in TripAdvisor Instant Booking had been an obstacle to participation.

The Priceline Group had never ruled out joining TripAdvisor Instant Booking despite previous criticisms about this lack of branding and effectiveness.

Booking.com’s participation, which will be evident in the U.S. and UK, where TripAdvisor has rolled out its Book on TripAdvisor feature, instantly gives TripAdvisor wider hotel coverage, including for hotels such as InterContinental Hotels Group, Hilton Worldwide and Starwood, which have boycotted TripAdvisor Instant Booking so far, preferring to focus on driving traffic to their own websites instead.

In June, TripAdvisor stated that about one third, or 235,000, of the hotels that are viewable on TripAdvisor could be booked on the site without having to navigate to a third-party hotel or online travel agency site. Although there is a lot of duplication, TripAdvisor expands the global reach of Book on TripAdvisor as Booking.com brings with it some 436,000 hotels.

Booking.com’s participation in Book on TripAdvisor is a significant victory for TripAdvisor, which launched its Instant Booking product a couple of years ago and was slow to get hotels, and especially big online travel agencies to join. The U.S. Justice Department was correct, during the Expedia-Orbitz merger review, to see TripAdvisor Instant Booking as an emerging player to heighten competition.

“Having The Priceline Group join the Instant Booking platform is a huge win for travelers and we couldn’t be more thrilled to have them as our first strategic, global online travel agency partner,” said Stephen Kaufer, CEO of TripAdvisor. “Millions of TripAdvisor users coming to the site to plan, compare prices and book their trip will be able to instantly book options from The Priceline Group’s global hotel inventory in addition to the 235,000 properties that are already instantly bookable on TripAdvisor.”

Darren Huston, CEO of The Priceline Group, sees the new agreement as an evolution of its longstanding relationship with TripAdvisor. “Our mission is to help people experience the world, and we are constantly innovating how and where we engage with consumers to help them do this,” Huston said. “We now believe this branded channel can help introduce more customers to our brands and the experiences we provide.”

For its part, the Priceline Group doesn’t view its participation in Book on TripAdvisor as any kind of game-changer but as a way to take better of advantage of one its existing marketing channels, namely TripAdvisor.

It will be interesting to see how Expedia responds to the Priceline Group’s agreement to participate in TripAdvisor Instant Booking, or whether Expedia will just keep focusing on its own priorities without an immediate counter-move.

In Expedia’s second quarter earnings call on July 30, CEO Dara Khosrowshahi talked about its exposure in TripAdvisor marketing, saying Expedia is losing shelf space by participating in TripAdvisor’s advertising auction but staying out of its Book on TripAdvisor offering.

“As far as TripAdvisor and Google building out booking capability etc. listen, this is nothing new,” Khosrowshahi said. “It’s something that has gone on for some time, as you know, as far as TripAdvisor goes. We do participate in the TripAdvisor marketplace as an advertiser. We are not participating in TRIP Instant Book. So from a theoretical standpoint, we are getting a smaller addressable market in the TripAdvisor marketplace and my guess is a lot of that market is going to the hotels direct. That’s not new. And that’s a headwind that we’ve been facing but it’s a headwind that we’ve navigated pretty well and we’re hoping to navigate on a go-forward basis.”

Khosrowshahi left open the possibility of Expedia participating in Book on Google rather than Book on TripAdvisor but the whole thing could be a bluff because TripAdvisor drives much more traffic to third-party booking sites than does Google Hotel Ads or the fledgling Book on Google.

“And in general, our business with TripAdvisor is growing,” Khosrowshahi said. “We do see Google make some moves in the area. And I’d say one difference that we see with the Google treatment of the bookings is that the Google treatment tends to be much more clear that the booking is actually with the OTA and Google is not the merchant etc. It’s really making the payment process easier rather than trying to own the customer one way or the other. So we do anticipate testing out with Google and testing out some of the other treatments and seeing how it affects our customers and our customer satisfaction and our economics in general.”

Booking.com will test to see what kind of payoff its participation in TripAdvisor Instant Booking will deliver. Then the next move will be in competitor Expedia’s court.

Photo Credit: TripAdvisor will get a major new participant, Booking.com, in TripAdvisor's feature that enables users to book hotels right on TripAdvisor. TripAdvisor