It's been clear for a long time that Instagram would become Facebook's (NASDAQ: FB) fourth billion-user platform, following Facebook, WhatsApp, and Messenger. Well, the popular photo/video-sharing service's user base has just officially crossed the threshold into 10-digit territory. While announcing its new IGTV long-form video service earlier today, Instagram also took the opportunity to confirm that it now has over 1 billion Instagrammers.