Bitcoin had a spam transaction problem ("dust transactions") which was a bigger problem than email spam, because every transaction is received by every node. It was easy to solve because Bitcoins are minted by proof of work.
I don't think a Captcha for signup would have been much friction. Certainly less than providing a phone number.
Why would someone want to use a closed messaging service like Signal unless they knew an existing user? I don't think that the requirement for that existing user to invite them would be a significant barrier. So I think it's not going to limit its reach.
I don't think a Captcha for signup would have been much friction. Certainly less than providing a phone number.
Why would someone want to use a closed messaging service like Signal unless they knew an existing user? I don't think that the requirement for that existing user to invite them would be a significant barrier. So I think it's not going to limit its reach.