At one point Facebook Official was a thing but not so much anymore. Since Facebook is where "old" people hang out online now, younger people are spending more time on SnapChat and Instagram. And now that Instagram is more popular than ever, Instafficial is taking over in 2018.
Now, if you want everyone to know that you are dating someone, instead of posting it on Facebook and declaring that you're a couple you go to your Insta and let all your friends know. The official caption on the post would be "IPO" or Instagram Post Official.