Not sure how they collected the data though.
https://www.broadbandsearch.net/blog/average-daily-time-on-s... puts it at 58 minutes, and https://www.nytimes.com/2016/05/06/business/facebook-bends-t... (from 2016) notes that it's 50 minutes (apparently reporting by FB in an earnings report).
(for those who don't remember https://www.theverge.com/2018/10/17/17989712/facebook-inaccu... )
Not sure how they collected the data though.