Why is @mazmhussain here, but not Jeremy Scahill?
Why is @bryanjohnson here, but not Andrew Huberman?
Why is @vgr here, but not Tyler Cowen?

("They knew someone here" is a fine answer, but more interested in why they stuck around)

I suspect a good differential diagnostic question is whether they quit twitter too

Only active on Twitter = You’re a High-Gravitas Man With a Plan™ and no room for error or humor
Also active on FC = The Plan™ has a low-gravitas FAFO thread 
Only active on FC = you have no plan, it’s all low-gravitas FAFO

Twitter is pure masochism at this point if you don’t have a plan that’d worth the pain.