As of December 2015, Facebook had 1.59 billion monthly active users, with millions more using the standalone Messenger app. When it comes to raw user numbers, no other network or messaging app comes close.
However, these numbers do not necessarily reflect users' feelings about a product. Users' enjoyment and perception of importance, as well as popularity among specific demographics, are not reflected in overall user number stats. So while Facebook may be the go-to social option for marketers playing the numbers game, there's a different service that should be focused on to reach the hearts of young people: Snapchat.