For event management, Framasoft created Mobilizon as a free and federated alternative for Facebook events.

From their blog article announcing this development:

From climate walks on Facebook to free software hackathons using Meetup : to change the world, utopians (like us !) too often organize themselves on the centralized platforms of web giants.

At Framasoft, we thought it was important to take the time to think about an alternative that could change the situation. We have just spent a few months, with the help of two designers (Marie-Cécile Paccard and Geoffrey Dorne) who haved interviewed many activists so as to better understand their digital practices. We looked at what a tool that would really empower individuals and groups could look like.

For project management, there is System-D.

I think both are directed to activists. They just don’t have the marketing budgets of centralized web giants so their reach is less important.

For messaging, several alternatives exist. https://gofoss.net/intro-speak-freely/

I also think that a lot of activists sadly do not understand privacy issues. They use Facebook as everybody else just because it works and they don’t have to pay money for it. If they don’t understand the issue, they will not search for a better tool.