You're right that captcha farms (human labor, usually close to slave labor) allow spammers a fairly cheap method of bypassing them, but they're fairly slow (unlike OCR), not necessarily that accurate themselves, and increases the barrier required to spam. If a spammer expects to personally make $4 per 1000 advertisements posted, and if 1000 captchas costs $1, that's a massive profit loss for them.
At least from my experience, setting up reCAPTCHA on personal sites reduces spam to 0. They'll usually only spend money on captcha farms for sites that garner huge amounts of traffic, leaving smaller sites in the clear.
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You're right that captcha farms (human labor, usually close to slave labor) allow spammers a fairly cheap method of bypassing them, but they're fairly slow (unlike OCR), not necessarily that accurate themselves, and increases the barrier required to spam. If a spammer expects to personally make $4 per 1000 advertisements posted, and if 1000 captchas costs $1, that's a massive profit loss for them.
At least from my experience, setting up reCAPTCHA on personal sites reduces spam to 0. They'll usually only spend money on captcha farms for sites that garner huge amounts of traffic, leaving smaller sites in the clear.