Jack Dorsey tweeted out to download the app back in June.

Edward Snowden has also backed it.

Signal is developed by Signal Messenger LLC, whose parent organization is the Signal Technology Foundation. They've just recently tweeted:

We are a 501c3 nonprofit. We're not tied to any major tech companies, and we can never be acquired by one either. Development is supported by grants and donations from Signal users.

Currently, the Signal Foundation's board consists of only three people: Brian Acton (who left WhatsApp/Facebook in 2017), Moxie Marlinspike, and Meredith Whittaker.

As for the question of trust:

Signal conversations are always end-to-end encrypted, which means that they can only be read or heard by your intended recipients. Privacy isn’t an optional mode — it’s just the way that Signal works. Every message, every call, every time. You can confirm that the server is operating correctly and that you are communicating with the right person using safety numbers.

The complete source code for the Signal clients and the Signal server is available on GitHub. This enables interested parties to examine the code for security and correctness.

Make of that what you will.

They used to have his endorsement on their website.