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Facebook dipped a big toe into the group-buying waters on Tuesday, launching a test of its Deals on Facebook service. But though it's coming late to the game, analysts and merchants say they won't be surprised if the social networking titan soon poses a viable threat to market leaders Groupon and LivingSocial.

Groupon has entered China via its investment in GaoPeng.com, the group-buying site that launched in Beijing and Shanghai yesterday.