Fediverse

I run BT Free public charity https://btfree.org/

I suggest putting it on your about page. i personally try to limit my time trying to contribute to FOSS. seeing that someone is a actual contributor helps prioritize how to try to help. i assume other people have a similar approach.

maybe Mastodon has the most users (idk)

Probably

can they get them out to vote?

My assumption is that if someone is willing to pay and support FOSS he is going to be more thoughtful. There is a risk that just newer platform might not be well known yet so they will get less votes despite being superior. maybe have people give a score on a scale of 1-100 like the ACSI and if a platforms say have one percent of the votes and has the highest average it is selected. if this will gather interest and will go on for a few years different approaches could be tried. and you could do polls asking the community what to do . you could set up some kind of paywalled community, i think discord and patreon has that feature but maybe there is a open source alternative (mitra?).

Nice job growing your instance BTW.