Out of all its apps, Facebook holds the highest number of monthly active users or in other words, logged-in or registered users on Facebook, Messenger, Instagram, or WhatsApp who used any of these Family applications one time either from desktop or their smartphones within the last 30 days.
The infographic below shows the last reveals of Facebook-owned apps including Facebook itself and according to it, the biggest social media platform in terms of active users is Facebook has 2.9 billion monthly active users.
Followed by WhatsApp with 2 billion monthly active users which were shared last year and the figure is now expected to have surged to several more millions or we never know, maybe another billion.

Infographic by: Statista.com