MarketWatch: Twitter is said to have quietly purchased Periscope, which has been developing a live-video streaming app. The social-media company is believed to have paid slightly less than $100 million, an amount that would make Periscope one of its more expensive purchases. The deal is believed to have closed about a month ago. Since then, Meerkat, another app allowing users to display live video from their phones on Twitter, has taken off.