Ok facebook is a legit site that raises no suspicions, but anything you write on Facebook is public.
I am not sure if there is any encryption on facebook groups, but I would really not trust it with my safety.
Plus, there is the matter of who you trust on those groups. Because anyone can leak the conversation, and then you really are a goner.
The tor project, also provides a way of connecting through snowflake proxies for those cases.
But if you run the risk of a random inspection for unauthorized software in your country, you REALLY should not be using the internet for any activism.
Or you should implement some elaborate dead man–panic button–kill switch, that would fry your hard drive.
Generally for activism tor is the go to method in countries with low democracy, but it should be employed with hardened software like tails OS, or something.
For the average user that does not like trackers on his browsing, and fingerprinting, by ad companies, Gemini is really better.
And combined with tor it is quite anonymous.