According to data coming from the company comScore corresponding to the month of June 2017, Facebook is the native phone app most used by users in the US. More specifically, 75.7% of the users have used the app at some time this month.
The second place is for the video app YouTube (67.2%), while another app stemming from Facebook, Messenger, was the third most used app (65,4% of the users).
The following graph shows the top 15 apps with most users in the US. The top 7 of the mentioned list are owned by 2 Internet giants: Facebook and Google.