Questions: What tactics could be used to move people en masse away from mainstream platforms

Keep letting large platforms run themselves into the ground. Has been working with twitter, each substantial change is causing more and more people to ask why in the hell do I wanna stay. Other then that just push those you know to use alternatives

I was able to easily convince friends to switch to firefox from chrome when they announced all those changes to their ad system

don't fight capitalist platforms with things they can match with money (advertising, cost, etc)

fight them with things they can't control, like kind/intelligent communities

I don't think its viable to post on Facebook (or whatever). with the goal of getting people to stop using Facebook. People on the site will quickly disregard whatever arguments put forward, because it is very hard to maintain credibility while you are on the site that you are arguing against. And if you do make an effort to maintain your credibility there, then you can end up having the reverse effect - in the sense that you are now a valuable contributor to the platform, and people will like and respect your contributions while staying on the platform.

Criticism one platform is more likely to be taken on-board if posted on a different platform though. For example, a lot of people on Reddit argue against Facebook - and I reckon it probably has an effect the people who read it. But obviously it isn't the ideal target audience. You really want to arguing against Facebook to people who are actually on Facebook!

I can't really think of a good way to get people to stop using particular social media en mass. (I'd written a couple of paragraphs about general strategies for changing people's minds; but I don't think it was helpful enough to bother reading.) I suppose the best option would be government regulation to undermine the targeted advertisement revenue streams. If these platforms were forbidden from collecting and using personal data for advertising, then the incentive for content-churn would be reduced. Without the money, the corruption would no long be self-sustaining. So political action to support strong privacy laws is probably the best way. Aside from that, probably the best way is to degrade the quality of the platforms. Don't contribute any content to them. Don't give them any kind of credibility by linking to them (for any reason, even negative attention). If you feel like being chaotic, I suppose you could create spam bots to just make the place worse, but that's probably not worth the effort. Actual spam-bots are common enough anyway.

What tactics could be used to move people en masse away from mainstream platforms

Propaganda, advertising and other kinds of informing about the issues of the mainstream platforms.

and more generally, do you think there is a point in it?

Yes. Not just because of privacy issues but also because the algorithm is made to filter what a user can see (making their source of information heavily biased and incomplete) and to addict them to the platform (this shouldn't be the main argument because people don't really care about it now but it can be a nice secondary one).

PsyOp them back.