The Facebook app has the most monthly iPhone users in the United States of any third-party app, and it is also the app on which iPhone users spend the most time, according to recent research from App Annie.
The report was based on April 2016 App Annie market data intelligence. The researchers looked at the which third-party iPhone apps (i.e., not pre-installed by Apple) have the most monthly active users and which apps have the longest average time spent per user.
Facebook Messenger has the second highest number of monthly active users in the United States.
YouTube is third, followed by Google Maps, Instagram, Snapchat, The Weather Channel, Pandora, Amazon, and Twitter.