What about the security of names of people who download the secure messaging apps Signal? Is there some master list that Signal keeps of every subscriber, that they could be forced to share, or could be hacked?
In theory, using secure apps simply identifies people sending potential sensitive information.
I've read here that "[Open Whisper Systems founder Moxie Marlinspike says] that [Signal doesn'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."
So the iPhone App Store does not publish this data, but this does not indicate whether they record the data or not.