Since opening its virtual doors last fall to anyone with an e-mail address, Facebook has graduated to more than 37 million users, and the 25-and-over crowd is its fastest-growing demographic.

A key factor in the site's rapid ascent: development of more than 3,000 free, third-party software applications that let Facebook "friends" trade everything from travel tips, Scrabble scores and books they're reading to hedge fund advice via a fantasy stock exchange.

The most popular of Facebook's 100-odd travel applications, downloaded by more than 2.6 million members since its launch by a freelance Web developer in June, is Where I've Been ? a map that highlights places users have been to, lived in and hope to visit. The interactive map includes a smattering of facts for each destination.

This month, rumors swirled - and were swatted down - that Expedia-owned TripAdvisor had paid $3 million for Where I've Been. The buzz surrounding the phantom sale to TripAdvisor, which offers a similar Cities I've Visited map on Facebook, shows the site "is certainly making on impact on travel," says Lorraine Sileo of PhoCusWright, a travel research firm.

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