[ansonparker]

The main flaw in this plan is the context in which Captcha's are generally used -- sign-up forms, comment forms etc.

They're part of a deliberate user action, not part of a general browsing behaviour.

People mid-stream in a sign-up form are very unlikely to follow an ad -- or if they do so, it might be at the cost of the primary activity.

I think it's for reasons similar to this that Facebook ads perform so poorly (apparently).

[jcl]

The point isn't to make the user follow the ad -- the point is to force the user to parse the ad as a step towards something the user wants. The theory is that this gets a message into the user's brain in a way that traditional advertisements do not.

While it looks like a captcha and appears in similar contexts, calling it a captcha is misleading: It fulfills its primary function even if it lets all the bots through.