Hi @webstationfactory,
Thank you for reporting this issue. I found some bugs 😛
When I entered the url of my site into https://developers.facebook.com/tools/debug/ , I found the facebook crawler blocked in “Logs” tab. It came from IE (Ireland) and the range of its IP address is 2a03:2880::/29 scanned by whois. In this case, the right answer is:
– Put facebook:IE or facebook:2a03:2880::/29 into “UA string and qualification” in “Front-end target settings” section.
But facebook:2a03:2880::/29 doesn’t work properly because of bug. So for now you can use facebook:HOST=2a03:2880 alternatively.
The range of facebook crawler’s IP address is my case. Please find your case in “Logs” and “Scan” tab. And I’ll also fix the bug in the future.
Please refer to “UA string and Qualification” for some details.
Thanks again.
According to https://developers.facebook.com/docs/sharing/webmasters/crawler , there are many IP addresses. So IP address based solution seems not to be feasible.
If we can retrieve AS Number from IP address, that’s the best. But Maxmind database supports only either of these.
So candidate solutions:
1. facebookexternalhit:!HOST
Certain UA and host name is unavailable. It’s almost equivalent to specifying only UA.
2. facebookexternalhit:SG, facebookexternalhit:IE, facebookexternalhit:US
Certain UA from certain country.
Dear all,
facebookexternalhit:* which checks only UA is also OK at “UA string and qualification” in “Front-end target settings“.
I updated “UA string and Qualification“.

Thanks.