Too easy to pick up feds / scammers / dealers / other predators looking for places to recruit fresh victims. Saw an acquaintance's "Art classes" idea fall to that last year: people would pay the $10 fee and then be disruptive with a crypto pitch.
I remember when Facebook launched I thought it might serve as this (eg, “oh h2odragon has a year old account and seems not like a psycho, I’ll let them come to my art class”). But Facebook wasn’t interested and stopped providing apis and whatnot. I think it didn’t align with their business model as they kind of want scams as it drives lots of revenue (in that a significant portion of ads are misleading or scams that would be devastated if people could post reputation info like “this magic mouthguard doesn’t work and just a $10 drugstore version wrapped in $40 of social media ads”).
The key is that there’s some public store of identities, and some level of social graph or complaints that lets you know if it’s a real person or not.
I think it’s better if it’s not blockchain and tied to some natural identity that people use- like Facebook- so there’s a virtuous incentive to keep it active. A blockchain wouldn’t really be self-sustaining as no one really uses identity as part of random stuff and it would need some way to pay for itself.