Business-focused social service Yammer has attracted a client base that includes 80% of Fortune 500 companies since its launch in 2008. Yammer officials attribute the network's success to a freemium model that lets any employee set up a Yammer account for his company, allowing bosses to see for themselves the value of having an in-house Facebook-style social network. "That is how we have gotten into 90,000-plus companies and organizations in just over two years," says founder and CEO David Sacks.

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