Expedia is the clear leader in the marketplace. Travelocity and Orbitz are bunched closely in second and third place. But Travelocity may be pulling away from Orbitz — by Merrill Lynch's estimates, Travelocity's share of bookings will increase by 2 percentage points this year.

Although some sites are clearly more popular than others in terms of bookings and audience, it's also clear that many online travel buyers visit more than one site to find the best deal. A Feedback Research study conducted in August 2005 found that over 60% of users who visited Hotwire also visited Orbitz, Expedia and Travelocity.

Travelocity still has a ways to go in the more fractured and competitive European market, where it is not among the top 10 travel sites (including travel provider sites), according to comScore Media Metrix. Merrill Lynch estimates that European online travel agency booking will grow 49% this year.

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