Expedia’s agreement on Thursday to buy Orbitz for $1.33 billion marked the latest in a spree of acquisitions it and the larger Priceline Group Inc (PCLN.O) have made to become the world’s dominant online travel agencies. Experts say the companies’ followings give them power when negotiating contracts with hotels and airlines, which to varying degrees rely on the sites to sell their products. Combined with Orbitz and Travelocity, which it acquired in January, Expedia received about 39 million unique website visitors in December 2014, according to Internet analytics company comScore Inc.