Something worth mentioning about CAPTCHAs is that they're really bad for accessibility and most users hate them.
There are better solutions for verifying that it's a person and not a spambot out there.
It's also incredibly cheap for spammers to outsource spamming to poor areas of developing countries and get armies of workers (including children) to do the spamming for them.
It's a complicated issue with no clear, easy solutions.