I believe strongly in the mission of decentralizing the web, so I have relentlessly annoyed everyone into using matrix and/or Signal (not perfect but better). You have to set expectations that you won't convert everyone and that FOSS has warts. For that reason I bridge everything and always meet people where they are and constantly show off to them that even if they sms/telegram/WhatsApp/messenger/slack/discord/signal me then I get it in one app with no tracking or ads on my physical phone that I carry in my pocket. Many people in my social circle hate telegram but have to use it, so bridging that was a big selling point for example.

I also periodically take advantage of news stories that trend like when discord was going to ID people, I made a point to bring that up to convert more people, and always get a couple new takers.

When something fails to work like group messages fracturing or big files not sharing well with iPhone users, I make clear the practical value of having our own control and resources as a social network.

I also try to make sure my server has useful bots, a small thing but every bit of utility and differentiation with proprietary services helps.

I share guides on how to get back in, one thing people get confused about is that they don't have to give PII but they do need to remember the home server URL and save their key and password. So making sure to have ready pointers and blurbs on how to login that you can share quickly is necessary because people inevitably get logged out, especially iPhone users get confused easily (for app store culture reasons I think) and need kind but eager assistance.

I try to make sure we have well moderated safe space e2ee rooms to talk about hard issues, and people do realize over time that certain things are more awkward to say digitally if you know it could be being surveilled or tracked, even the metadata.

I've also encouraged technical friends to run their own servers and federate, got a couple to actually do it which is a cool feeling.

It takes time (years), patience, and effort, but many little pushes will build a community.