Comment by GuB-42

Call to donations, ads, pre-mined cryptocurrencies, selling cosmetics, premium features no free service offers, partnering with other organizations, etc...

They already do some of these, and some are less popular than others, but the key is to keep the essential features free and easy.

On Discord for instance, a free account is enough to cover all of most people needs, but you get a little extra by paying a subscription, and it is enough for Discord to be worth billions. Maybe not the perfect example since Discord has a critical mass, but no one wants to leave just because they don't have premium features (larger uploads, higher resolution streaming, flashy emoji) for free.

For Signal, it seems like just calling for donations is enough. They have a good image, so they can do that. It can actually be a solid business plan, look at Wikipedia, they get more than $100M a year doing that despite the controversy.