Android solved that UX issue with Chrome custom tabs (https://developer.chrome.com/multidevice/android/customtabs). It uses a seamless UI exiting the app and everyone uses the up to date browser on the phone.
On the current Android and iOS implementations a user is far less likely to share on, say, Twitter, because they won't have their Twitter login cookies in the Facebook browser. I suspect Facebook is quite happy to keep things that way.
Otherwise, CCT and SVC should be the way to go, still much better than FB. Aside from tracking and all, it doesn't get the niceties of a full blown browser.