There's a new change to Facebook's mobile apps that means whenever you tap on a link you'll see the resulting webpage open up in the app's own stripped-down browser window. Facebook does display a brief warning — and claims that this new method is faster all around — but if you want to turn it off then you need to toggle a setting inside the app.
Unfortunately, for the time being, you can only disable the behaviour on Android. From inside Facebook, tap the menu button and choose App Settings, then select Always open links with external browser from the menu that appears. While you're here you might want to switch off auto-playing video adverts as well, but that's up to you.
No such luck on iOS where you're going to have to put up with Facebook's default browser app for now. If you want to see the page you're viewing in Safari instead, you'll need to tap the Share button and then choose Open in Safari from the menu. Not ideal, but one way of escaping the Facebook environment for a few minutes. (You can still switch off video auto-play, by the way — it's on the Facebook page in the iOS Settings app.)