I've had success and you're right that it is about network effects. So you gotta take into account who you can convince first. But also consider that Signal hasn't been fully featured until about last month. So it isn't a good idea to just try to convert random non-techy people. For them you'd need to use current events that highlight how important privacy is (which there have been quite a few this year). But also focus on generating a critical mass. Now it isn't hard for me to convert people because we'll be planning things and 4/5 people have Signal so you just strong arm the fifth person and then they start using it more because they realize a fair amount of their friends are already there. It takes time though and let's be real about that Signal hasn't been fully featured. Until recently it has been more a geeky app.

So tldr target the people you want to convert to develop a critical mass.