MoPub, the mobile ad company that Twitter, Inc. (TWTR) bought for $350 million, only had $6.5 million in revenue in the first half of the year, following $2.7 million in net revenues for 2012. While most analysts have been shocked by how small the company turned out to be, Pivotal Research Group’s Brian Weiser thinks the investment is best understood as a strategic move that expands the types of ad products that Twitter can offer and hopefully give it a strong presence in a growing market instead of a straight value proposition. Twitter’s strategic goals with Mopub “Twitter’s choice to…