Earlier quoted context omitted.
From their description in the iTunes store: 'Pay Nothing - Signal is supported by a team of dedicated developers, community donations, and grants. There are no advertisements, and it doesn't cost anything to use.'
So it's a project, not a business.
When ideals are involved (strong encryption, privacy, ad-free), being run as a lean, open-source project is worth much more than being owned by a megacorp like Facebook.