Twitter Inc. agreed to acquire mobile-advertising exchange startup MoPub Inc., in a deal that adds real-time bidding to the microblogging service’s ad platform.

Twitter paid about $350 million in stock for MoPub, in what would be the largest acquisition for the San Francisco-based company, said people with knowledge of the purchase who asked not to be identified because the discussions were private. The deal was disclosed in blog postings from both companies yesterday, and technology blog TechCrunch earlier reported the acquisition and price.

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