Users are reporting errors to site status tracker DownDetector.
A huge number of users have been signing on to Signal and the app was downloaded by 17.8 million users over the past seven days.
Brian Acton co-founded WhatsApp before selling it to Facebook and then he co-founded the Signal Foundation.
WhatsApp postponed a data-sharing change after many people concerned about privacy fled the Facebook-owned messaging service.
The company assured that they will clear up misinformation around privacy and security.