Signal’s president reveals the cost of running the privacy-preserving platform—not just to drum up donations, but to call out the for-profit surveillance business models it competes against.
One of the lesser known, possibly reformed(?), Facebook billionaires gave them a bunch of money several years ago, and they've been trying to bootstrap that into long term sustainability via donations from regular people.
Edit: See duplexsystem's comment below for a better summary.
Brian Acton, the creator of WhatsApp, co-founded Signal and loaned them a shit ton of money. "$105,000,400, due to be repaid on February 28, 2068. The loan is unsecured and carries nil interest." He's still Executive Chairman.