As per my Google Analytics data, most people on my website come from:
m.facebook.com / referral
Are these people coming to the website from the Facebook iOS/Android app or m.facebook.com or both?
They come from anything Google considers a mobile device. You can see device specific info in the analytics pane- more info here https://blog.kissmetrics.com/what-analytics-is-hiding/
Google Analytics interprets the FB App as a visit from the FB mobile site...so it should register in your analytics visitor activity as a visitor coming from m.facebook.com