Comment by creata

> you'd use a proof-of-work scheme

I thought the general belief (e.g., '“Proof-of-Work” Proves Not to Work') was that proof-of-work isn't very good anti-spam.

> or a Captcha

Aren't bots better at those than humans by now?

And making people do captchas in an instant messenger is a great way to make people not use that instant messenger.

> or an invite-code system like lobste.rs or early Gmail.

That's not a long-term option if you want to make something mainstream.