Did you relogin to the SOGo web frontend after creating that web calendar?
SOGo only synchronizes web calendars on login.

Yeah but my URL is not finishing by .ics and that's why it's not working because it's working perfectly with another shared cal url (from google by exemple) which is finishing by .ics

I've just tested with an iCalendar file that is NOT finishing with an .ics extension and it all worked fine.

I suspect the output from Facebook might be incorrect. Try to produce a sample that works elsewhere but not in SOGo and attach it to this ticket.

Also try to retrieve it with wget.

I can reproduce it. Not with Facebook but with a calendar not finishing by .ics. Simply a file named "ical". Valid on Google Calendar but does not show anything in SoGo.

Maybe it helps so here are the response header of that url:

HTTP/1.1 200
Strict-Transport-Security: max-age=31536000
Cache-Control: private, must-revalidate
P3P: CP="CAO PSA OUR"
Last-Modified: Tue, 19 Nov 2019 22:39:30 GMT
Expires: Tue, 19 Nov 2019 23:39:30 GMT
Pragma: private
Content-Type: text/calendar;charset=UTF-8
Content-Language: en-GB
Transfer-Encoding: chunked
Content-Encoding: gzip
Vary: Accept-Encoding
Date: Tue, 19 Nov 2019 22:39:30 GMT