What is the single biggest reason/motivation/urge to join Web3 social media platform besides MONEY?? 

Lets be honest, dont hold back. 

Share, let the community join so that find a some solution to make it sustainable.

Progression & embracing future technology & adopting to the new Era that is not here yet.

Is it enough to onboard mass people? Will it be strong enough to sustain in long run? Or appeal mass adoption? We need few more people to have a disscussion.

For now it's not enough. Something really drastic must happen for good or bad so it will impact over the mass .
BTW, did you see the movie: 
"THE CONGRESS" ( with Robin Wright )
???
Pls, watch this movie and than watch the movie: "Existenz" (stylized as eXistenZ)".
And than watch: Ready Player One.

After that, I'm sure you'll have a proper answer ...