The vast majority of FB uses are using an app, not a browser. clicking a link in the FB app does not take out "out of Facebook" they know if you clicked the back arrow within the app, and went back to your feed.
Thanks for the link. I was underestimating the impact of mobile. But I believe the parent to my comment was overestimating, and I'm kind of sick of the "everyone uses the web on mobile anyway" meme.