[3np]

I'd start with fine-combing the terms of use and service in place at the time of you agreeing to them (web.archive.org is your friend). Chances are you're SoL but always start there.

As for actions, consider if maybe taking back some control is better than yielding to someone else for next time if control is what you actually desire? There are various federated and/or self-hostable alternatives with way better migration stories.

Or maybe offer meetup lots of money for it back. Not much else you can do.