When Facebook (No. 341 on the Fortune 500) acquired Instagram (whose employees are pictured here) for $1 billion in 2012, critics squawked that the price was outlandish. Today the deal is viewed as a bargain. The company’s app, which lets users apply stylized filters to photos and share them, has bolstered Facebook’s mobile presence, a former area of weakness. And the youth of the average Instagrammer is seen as a boon to its parent. The photo service more than doubled its user base in 2013 and today has more than 200 million active users (two-thirds of them outside the U.S.) per month, just one of many factors that contributed to Facebook’s (FB) $7.9 billion in 2013 revenues.