55% of all web traffic worldwide comes from mobile.
https://www.statista.com/statistics/277125/share-of-website-...
And considering that most people spend 8 hours a day at work, how much personal usage on the web comes from the desktop? How many younger people are using computers for personal use - especially if it a shared family computer.
Heck I am a software developer and my personal computer is just sitting in a corner as a Plex server. I haven’t used it for anything productive in a year and a half and that was for updating my resume.
Besides that, Facebook got more popular because of mobile when everyone had a camera in their pocket with GPS when they csn post the highlights of their life. Not to mention WhatsApp is all about communicating real time and Instagram is about sharing pictures.