Snapchat - does it even exist anymore? It had its time, but personally I don't know anyone still there.
Same with Facebook Messenger, which has completely been replaced by WhatsApp in my (non-nerd) circles.
I thought about including TikTok but it would be more in the
- YouTube, TikTok, Vimeo, PeerTube
category. To me, these are borderline social media and more akin to television - pure consumption, heavily discouraged participation. Just look at the comments on any of these for a proof.
I'd give you Instagram, it seems immensely popular and intensively used as a communication platform.
If we disregard our own bubbles and go strictly by popularity we should have considered WeChat and Douyin too, I guess.