Groupon.com leverages its 2 million subscribers to offer daily deals and discounts, forming a nexus of social networking and online shopping. Companies use the site to boost awareness of their brand, and to piggyback on the site's social-commerce infrastructure, experts note. "It's commerce layered on top of the social graph," says company founder Andrew Mason. "It's a method of buying things that's propagated over sharing tools like Facebook or Twitter, without which it couldn't exist."

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