Instagram announced Wednesday that it now has 300 million monthly active users, up 50 percent in just nine months. That makes the service, a photo- and video-sharing app owned by Facebook, more popular than Twitter, which had 284 million monthly active users as of the third quarter. More than 70 percent of Instagram’s users are outside the United States, the company said. Instagram’s rapid growth, while impressive, is hardly surprising. Visual posts, and especially video, have become the hottest growth area in social networking. Facebook has been changing its news feed over the past year to better showcase video content, and Twitter is focusing more on building up its Vine short-video service. Advertisers are also demanding more video options, and online services are hustling to meet that demand.