The anecdote is supposed to show why one can't just write their own app. It doesn't work that way.
Consider that you're a distant outlier.
You just naturally remember, like you would remember where your friends live. I guess if you are constantly dealing with "contacts" rather then friends it would get confusing.
I use
1. WeChat
2. WhatsApp
3. Threema
4. Frozenchat, a Jabber client with OTR support
Don't have signal yet.
Ah, and I can "chat" via Text message too :-)
2. global leader right now
3. as compromised as whatsapp,has data compliance with multiple countries
4. no name fork of a no name
2. correct
3. Multiple countries? You would need a court order from Switzerland. I am not even sure they can access your messages if they wanted to.
4. Irrelevant because it is a client to an open protocol. Which client you use does not matter. I just named mine.