My solution, but see if it is appropriate for you, was to drop all friends I was in contact with through Facebook (or any other application you are currently using to contact them). The good side is you will get more time for productive things. Funny fact, I think 3 years later I messaged a friend who finally took the time to install a new application because he wanted to pick up a woman who used it. Bottom line, the carrot to make friends install Signal can be sex with a woman. My best friends did install Signal though.