The Signal messenger and protocol.

I don't personally use Signal but I do use a bunch of other tools that the average user balks at (Linux as my daily driver, vim, Libreoffice, a god damned password manager, etc...). Call me defeatist but after years of experience evangelizing to people about these sorts of tools, I've written it off as a lost cause. Most users only care about one thing, "what actual problem that I have does this solve?" -- I'm not even talking about the other uncomfortable conversation of "if I seriously use this what new problems will it create for me?". The median user won't budge on account of a promised solution to a problem that's never bothered them and they've never thought of until you brought it up and insisted that it was totally a big deal.

When the normie alternative is literally on fire that's when you get people willing to make changes. So, if you want to get a lot of people to use Signal, all you have to do is orchestrate a series of huge damaging scandals that people could totally imagine happening to them, and that were only possible because those involved were using more mainstream messaging apps. Short of that I have no idea.