Freenet uses a similar technique against spam (Score 1)

Freenet uses similar techniques against spam on the Freenet Messaging System (FMS).

Two things are mentioned in the article: many eyeballs, and moving to a pull technology from a push one.

FMS uses a web of trust, similar to PGP's to rate the trustworthiness of users, and this makes it simple to do collaborative filtering of spammmers (many eyeballs).

It also uses a pull technology, where each user has their own message queue, and you poll the queues of people you trust. There are tricks to make this scale up, so you don't have to be polling millions of people all of the time.

Initial entry to the web of trust is done mainly through a captcha system, although it can be done through any out-of-band method. Even if the captchas are defeated, which they will be regularly since this is an arms race, the first two steps should mitigate the damage done, by rapidly spreading bad trust values for the spammer to other users before they get to downloading their messages.

It works well in practice on a small scale, but obviously there are neither the numbers nor the dedicated spammers to test it out properly.

If anyone wants a challenge, please come on Freenet and try to spam the Freenet Messaging System!