TechCrunch reported on September 9, 2013 that Twitter bought MoPub for $350 million worth of stock. MoPub is a startup focused on enabling mobile publishers to manage their ad inventory. Twitter and MoPub have confirmed the acquisition though not the price.
This is another step in Twitter's push to monetize its user base, this time on the mobile front. According to TechCrunch, "Last year, Twitter started to make more money out of advertising running on its mobile platform than it did on its desktop product, according to this report in the WSJ based on stats from eMarketer."