I mean, Signal is open source and not owned by Facebook, so I'm not sure why anybody uses WhatsApp instead.
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.
Most new social platforms that make it big don't really take over older ones, they just grab a younger generation - usually just by being the network their parents aren't in.
It's network effects
In my own anecdotal experience Signal ranks way below Viber (popular with migrants + expats), Wickr (popular with people doing illegal things and corporate executive scheming), Telegram (popular in crypto, scammers and terrorists)
The only real broad use of Signal i've seen is amongst journalists - and even there i'm not certain how much they actually use it or if it's just the "i'm crypto aware" version of a blue checkmark for their Twitter profiles
Signal's lack of a web interface is another reason.
Moreover, Telegram took the users that left WhatsApp for more secure alternatives (even though Telegrams homemade encryption doesn't look promising).
Same reasons why a lot of people don't leave Facebook.
A lot of Signal is basically "trust Moxie".
Let me come out and defend Signal (I usually defend Telegram, but I don't think we should be unfair to anyone):
As far as I am aware no one who knows what they are talking about has come out with anything that says Signals end-to-end encryption is broken.
If I have understood it correctly an as long as that is true, NSA, FSB and the Chinese might be running the message handling together and there's still no reason to be worried that your messages will be intercepted in transit.
Disclaimer:
- as far as I am aware Signal is the safest messenger available for everyone
- even if all the above is true you are still trusting them with your metadata. I think they are good people. If you are scared of them, be aware that they know who you talk to and when. This is however true for any mainstream technology as far as I am aware.
- being good at crypto doesn't make them immune to bugs. There was a nasty vulnerability a few months ago that was remotely exploitable. Again, this is the same, or even worse for every other messenger.
You have to check xmpp with omemo, it has libre servers and in federated
It's too bad bitmessage can't scale :/