TripAdvisor is in an expansive mood. In July, it acquired two small startups, VirtualTourist (user-gen travel guides) and OneTime (booking price comparison). And today it announced a majority investment in FlipKey, a guest review site for vacation home rentals. The amount of the investment was not disclosed.

FlipKey launched only last March. It covers 50,000 vacation rental properties in the U.S., which CEO TJ Mahoney says represents a $60 billion market. So it is a pretty big niche. FlipKey aims to become the reputation management system for vacation home rentals. Property owners can take review data in the form of a widget and place it on their sites or property listings. (See examples here and here).

TripAdvisor plans to include FlipKey reviews on its own network of sites, which attract 12 million visitors a month in the U.S., and 28 million worldwide, according to comScore.

Mahoney previously was a co-founder of Compete.com. Brightcove CEO Jeremy Allaire is an advisor to the startup. FlipKey raised $500,000 last December from angel investors, including Allaire, Care.com CEO Sheila Marcelo, and venture capitalist Nick Beim of Matrix Partners.