The sweeping success of online social networks like MySpace and Facebook has created new avenues for businesses - from New Balance to TripAdvisor - to market themselves.

Since Facebook opened up its platform to third-party users in May (allowing anyone to offer a free tool or game for Facebook users to download) Web traffic on the site has taken off at warp speed. Last month 33.7 million US Web users visited Facebook, up 46.5 percent from April, according to comScore Media Metrix, a firm that tracks Internet traffic.

One month after Facebook allowed outsiders in, TripAdvisor, the Needham-based online travel guide, launched an application called "Cities I've Visited," which the company's chief executive, Stephen Kaufer, likened to the maps children used to festoon with pins showing the different places they had visited. At last count, over 2 millions folks had installed the application on their profile page, Kaufer said.

"We certainly considered other social networks but what's particularly appealing about Facebook now is the viral nature of the news feed," said Kaufer. When someone installs an application, all of their friends are notified. And each time they update their profile friends are again notified which, Kaufer said, raises their curiosity. "Of all of their friends, some portion will check out and install the application, which in turn, causes more people to notice."

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