Slack's value doubles to $2.8B in six months By: CNBC.com News April 16, 2015 at 13:38 PM EDT Six months ago, a year-old messaging app called Slack lit up the tech blogs when it raised money at a $1.2 billion valuation. Well, try $2.8 billion Read More >> Related Stocks: Adobe Systems Expedia Google, Inc. New York Times Company