I don't use Facebook myself so haven't encountered the error, but I can at least load the Instagram home page with the "(a)" user agent without issue. It would seem that Facebook has fixed the issue.
I'm pretty sure they took a long time to prepare this update (patch) since they discovered apple devices were being actively exploited. Now Apple device owners have to wait even longer until they fix the patch and publish it again.
So much for browser security.
This fixed Facebook for me but Instagram on desktop Safari won't allow me to sign in and just shows "Sorry, this page isn't available." Anyone know what might be going on?