I partialy replied on your other post Feature request : Be able to create a new event by importing it from a connected calendar (or ics file) - #3 par Lahax
I’m part of the maintenance team since 2024 when Kaihuri took over from Framasoft.
If I’m correct, Framasoft planned to create some official connexion between a facebook group and a Mobilizon group, which would need to be approved by Facebook. This was not pushed much, and unfortunately we won’t have time and money to invest on that this year. But if somebody would feel like working on that, they would have our full support.
On another end, Mobilizon Import tool https://import.mobilizon.fr/ is a live thing, you can feed it with a URL or ICS feed and it extracts whatever data he finds. I’m nearly sure that it used to work for facebook, but I just tried now and it doesn’t. I guess that Facebook has changed the way they display their data or something 
Maybe we can make it work again soon.