In short, if a government demands that Open Whisper Systems hand over the content or metadata of a Signal message or a user’s contact list, it has nothing to hand over. And that government will have just as little luck requesting backups of Signal messages from Google or Apple.
From a user privacy perspective, Signal is the clear winner, but it’s not without its downsides.
Compared to WhatsApp’s 1 billion users, Signal’s user base is minuscule. Marlinspike said that they don’t publish statistics about how many users they have, but Android’s Google Play store reports that Signal has been downloaded between 1 and 5 million times. The iPhone App Store does not publish this data.
Too bad almost nobody uses Signal. Convincing people to switch their text-messaging app is almost like trying to get people to convert religions.