alt Hacker News(Disclaimer: I've never tried to move large numbers of people off of Facebook; I have organized community groups from scratch before, and I have led initiatives at work that consisted largely of convincing people to do a thing. Much of this advice is from that perspective. YMMV.)
So: my advice is to not think of it as all-or-nothing. You will not be able to move 300k people off of Facebook overnight. This is somewhat akin to every IT migration project ever: it always takes longer than you think, and is not always a linear process from "fewer people migrated" to "more people migrated".
It's also akin to community organizing: there is no substitute for actually talking to people about it, especially in the initial phases. Or: high-touch sales, where you may initially need to spend a lot of energy and time per person successfully moved over. The other common thing here is that you will hear "no" a lot, which is a valuable experience anyways (but will be frustrating).
Also: unfortunately, no one will care if it's self-hosted or federated, outside of niche tech circles. They will care about whether they can reach the people they want to reach, and whether the user experience is good or not. This is reality: talking about these points will not help you.
Some things you'll probably need to do:
- Identify a single credible alternative platform. - Identify specific groups of people who are willing to be early "de-adopters". For instance: a local youth group, a sports club, whatever. Ideally you are a part of this group already; you then have a much better chance. Businesses will likely say no, so you want community groups. - Within those groups, identify champions: people who care about the same thing you care about, and are willing to commit time and effort to help. - Together with your champions, build a toolkit that allows you to scale up your efforts. This may be guides on how to talk to people about the change - what works, what doesn't. This might be instructions for setting up a specific platform. It might be communications channels, leaflets / flyers for putting up in public places, whatever.
The first thing that comes to mind for me is
https://nextdoor.com/
which is very much about community organizing but it has an aura of "people spreading rumors about bicycle thefts at the movie theater downtown (why don't they call the cops?)", the woman who radiates creepy signs of precarity (is cleaning up and looking for the phone number of the people who are suspected to run an illegal landfill) and then posts screen shots of the creepy come-ons she gets from guys who want to be her sugar daddy, etc.
Maybe there's a space for a platform that specifically targets small, community, in person kinds of organizations, maybe even targeted to a particular geographical area; something like Meetup but just a little less structured.
Here's a fair sized local organization (has more than one run a month) that has a good site
https://fingerlakesrunners.org/
But making that scalable is tricky; somebody in the club's leadership is a Wordpress pro. $5 a month would be cheap, but people are niggardly. If you're a web tech native owning a domain name is table stakes, but I think you'd lose 80% of "normies" even the phone-dependent "internet natives" if they had to get a domain name. There is a certain amount of panic over the breakdown of community organizations, see the line of research described in this film
https://www.rottentomatoes.com/m/join_or_die
and rather than getting $5 a month out of people who think they can't afford it, getting funding from somebody like the United Way (for a particular area) or the Knight Foundation might be a better idea.