Because Facebook acquired WhatsApp we never got to see what would have happened with WhatsApp as more and more people discovered it. WhatsApp was charging users some token amount like $1 before it merged with Facebook. Could that sort of small donation model work to keep a service like WhatsApp (or Signal) running? If 50 million people donate $1 to Signal, does he get his $50 million donation refunded. Shares of FB are down^1 while shares of SIGL are up (again).^2,3

1. https://finance.yahoo.com/quote/FB/history?p=FB

2. https://finance.yahoo.com/quote/SIGL/history?p=SIGL

3. https://news.ycombinator.com/item?id=25770464 <-- Suggested reading before replying ;)