Video game streaming site Twitch hit a milestone with 100 million monthly average viewers by the end of 2014, more than double the audience it had the previous year, the San Francisco company said Thursday. Twitch, which lets video game players broadcast their games live online for others to watch, also had 1.5 million unique broadcasters per month, up from just 90,000 at the end of 2013. The Seattle online retailer paid $972 million to buy Twitch Interactive, with employee retention incentives boosting the total price to about $1.1 billion. Twitch’s success prompted game developer and distribution company Valve to launch a competing service, Steam Broadcasting, this month. The company ended 2013 with about 45 million monthly average viewers, but its popularity surged as more gamers bought Sony PlayStation 4 or Microsoft Xbox One home game consoles with a built-in Twitch app. Twitch broadcasting is now built into some mobile iOS and Nvidia Shield tablet games.