You might be interested what https://hcaptcha.com is doing.
First, it seems tacky scrounging for peanuts from the users' captcha work. Or it's like a product/services website showing Adsense ads. It's a cheapening message to send.
Second, since you make more money from more captcha volume, you're incentivized to maximize your use of captcha which is at odds with every complaint in this comments section about captcha. Most sites only use captcha to gate low-volume actions like register/login (e.g. HN).
They created their own Ethereum token too which always puts a bad taste in my mouth these days.
Finally, it doesn't address the upstream complaint that someone else is profiting off the user's "work" rather than the user. Though I don't find that complaint very reasonable. And a tiny fraction of a cent sounds about right. The truth is that users benefit from anti-abuse systems. The number of bots that HN's recaptcha on register/login has stopped is worth that tiny fraction of a cent to most users.
Sites can set the difficulty level necessary for their application. Some are under continual targeted attack, others are mainly keeping out rogue automated spambots from their comments section.
The user is typically getting a free service, a better site experience due to less bot traffic, or both. I think sharing the value of their work with the website is a fair deal.
As for using blockchain tech for ledger functions, that is all under the hood: websites can cash out to dollars as they prefer.
(disclosure: work on bot detection at hCaptcha.com)
Yes, mainly because we're talking about fractions of cents. Also, it's not for free; the website and its users get a good anti-abuse measure in return.
There's a big difference between something that cannot make money and something that makes pennies for the site. But, to be fair, 99.9% of users aren't going to notice the difference in captcha branding either way unlike my example of a banner ad on a retail site.
My main reaction is that the UX incentive to minimize user exposure to captchas seems to work against the primary pull of using hcaptcha in the first place.
Though one site I can think of that has a captcha behind every action (every post) is 4chan. Maybe you can get them on hcaptcha one day. It would at least help you test your tagging system against vandalism. :)
I didn't find any pricing examples on hcaptcha's website. For all I know, people are bidding 5 cents per image.
Anyways, I definitely want to see more serious contenders in the captcha space so that we all aren't contributing to Google's middle-manning of the entire internet, and I'd like to try hcaptcha even out of curiosity.
If it means no/fewer ads to support a site then the user benefits because they don't have to pay real money to keep the site up.
If you use my referral URL I get a bump in the queue: