>If/when Signal becomes big enough to be used by a lot of people

I think people (including the government) noticed it a long time ago.

From 2015:

'According to CNN, "[t]he same technology that keeps a conversation private between you and a family member also gives a safe haven to a terrorist in Syria and the person in the United States he's trying to recruit to commit an act of mass murder."'

https://en.wikipedia.org/wiki/Signal_(software)