Facebook (NASDAQ: FB) has four platforms that now have over 1 billion monthly active users (MAUs): Facebook, Messenger, WhatsApp, and Instagram. Instagram was the most recent to hit that milestone, which occurred just last month. For social networks, user metrics are among the most important operating metrics for investors to keep an eye on, but Facebook only regularly discloses user metrics for its core Facebook platform.The other three platforms tend to only announce when they hit certain milestones, instead of providing investors with regular updates.