I have looked several times and not been able to find anything. Facebook knows people want this and they know they don't want you to have this. They dedicate resources, legal and technological, to ensure that the only way to interact with Facebook is through their designated data-mining channels.
My advice would be to let the organization know that you don't have a Facebook account and if they want your help to volunteer that they need to use a platform that doesn't exploit it's users for profit. It's a totally unreasonable expectation.
Make your objections known, offer to help the transition, but it's likely worth it to accept the tradoff and participate via Facebook when they don't make effort to change.
maybe - it's my local circus and they're very nice - the thing is, a lot of the other volunteers are not tech savvy, and likely won't change to a new platform unless a lot of energy is put into making the transition easy - but it might be a project for the future :))
Can you DM a bit more details? Depending on their size and budget, I can help. I can host their own Mastodon/Matrix server for less than $30/month and provide the tech support to the transition.
Tried that for messaging once. It worked for like 2 days until facebook decided someone had "hacked" my account and I needed to reset my password. Didn't help that the matrix bridge was very flaky and nigh impossible to set back up again after it failed, without just deleting the entire matrix database and starting from scratch. Not recommended.