Former Microsoft Corp Chief Executive Steve Ballmer has taken a 4% stake in Twitter, according to his spokesman, making him the third-biggest individual shareholder in the social media company. Ballmer's stake is worth more than $800 million based on Twitter's $21 billion market value. Only co-founder Evan Williams and Saudi billionaire Prince Alwaleed bin Talal have greater stakes among individual investors. Shares of Twitter rose 5.6% to $31.34 on Friday, hours after Ballmer tweeted from a non-verified account that he built up his stake over the past several months. His tweet lauded Twitter's new 'Moments' feature, which curates the best tweets of the day, and Dorsey's appointment as permanent CEO last week. "Good job @twitter, @twittermoments innovation, @jack Ceo, leaner, more focused," the tweet said.