Not long ago, Facebook rolled out it's in-app browser for Android that opens links from within the Facebook app--so you never have to leave Facebook. If you'd rather open sites in your mobile browser of choice, there's a setting for that.
As MakeUseOf points out, Facebook's in-app browser can be faster than other browsers (e.g., Chrome) to load, but Facebook's browser doesn't have many sharing options other than sharing to Facebnook or copying and saving links. I save most links to Pocket or share via email, so this lacking feature is a big dowside to opening webpages in Facebook's app.
To turn off the in-app browser, all you have to do is tap on the Facebook menu icon in the top right, then App Settings and then chck the option to Always open links with external browser.
Now Facebook will remain simply a social networking app and your browser the app you view webpages in.