It looks like Facebook is again testing a new bit of functionality in its Android app with a subset of users. After the most recent update, people are suddenly seeing a built-in browser that loads timeline links rather than booting you out to a full browser. Whether this is a good thing or a bad thing depends heavily on how you feel about the Facebook app in general.
The browser appears to just be a WebView like many other apps use.
The Facebook App Has Started Opening Web Links With A Built-In Browser For Some Users was written by the awesome team at Android Police.