Whenever I get a new phone, one of the first things I do after installing all of my apps is head into the Facebook app settings and enable the 'Links open externally' toggle. However, Facebook may finally be adopting Chrome Custom Tabs, three years after Google first announced them.
Several readers have sent in tips showing that the option of selecting Facebook's browser is gone from settings, and that links are now opening via Chrome Custom Tabs. Facebook put 'Save link to Facebook' and 'Send in Facebook Messenger' in its implementation, which makes sense.
Interestingly, this only seems to be on certain versions of Facebook (not the most recent). My Pixel 2 XL is on Facebook v126.96.36.199.11 and my Pixel XL was on v181.x, but both still had the 'Links open externally' toggle. I installed v188.8.131.52.75, which a tipster had on his phone, on my Pixel XL. Chrome Custom Tabs then started working on my Pixel XL, despite the fact that the links toggle is still in my settings.
So if you want to try Chrome Custom Tabs in Facebook out right now, grab the Facebook v184.108.40.206.75 APK from APK Mirror.