I have and use Signal with some friends, but there are also loads of people I communicate with who couldn't even tell you what open source software is, let alone articulate a preference for it over good UX.
Are all of your friends software engineers and/or technophiles?
Edit in case of potential ambiguity: s/a couple of/2
When all your friends are already using Facebook and you start telling them to use Signal instead... well, I can tell you from experience that it's almost impossible to break the status quo.
What has happened to me is that usually there's 1-2 persons from each friend group who care enough that they will relay information to you through Signal.
I still don't have a friend group that is 100% Signal. For that to happen, more than 50% of the group would need to care enough about privacy to completely abandon other communication channels and accept the cost of switching platforms. The rest would probably follow. In reality, I don't have a single non-tech friend who would give a fuck about encryption. You tell them about Signal and they go "cool", that's it.
Plenty of us still prefer using command-line to this very day. Most of my work is still done on DOS 6.22.