I think Zuck, having taken the market from Orkut, Friendster and MySpace, decided he will not let anyone take it from him, no matter what.
E.g. Skype was still on every phone in 2014; it’s not about install base - it’s about potential to become a social network, which WhatsApp had, and Skype didn’t.
WhatsApp is a social network because of groups. I think I never had a group with Skype. I don't even know if they are a thing there.
I'm currently using WhatsApp and Telegram groups to chat and exchange pictures and links with groups of friends and colleagues. I'm trying to move from WA to Telegram starting with the most technical friends (as always has been with new things.) I'm basically not using Facebook anymore. I appreciate that those apps let me partition people in groups with well defined boundaries. I believe FB has something like that but the UI was more about posting something and let all contacts see it.
Anyway, everything I post there is meant to be lost. I'm backing up images, messages not so much. Definitely not on WA which is pretty much hostile to backups as it doesn't give us the key to decrypt the local databases. Maybe it is not to break E2E encryption and not let other apps decrypt messages, but I should be able to download the key from somewhere if I ask for it. Uploading to Google Drive an unencrypted database is not good.
On the other side Telegram has a kind of distributed database on all my devices. Probably bad for E2E encryption unless every devices has its own key and messages are encrypted for all of them, or another equivalent system. I didn't investigate how it works.
Also Facebook has only been top dog for a little over 10 years.