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How is a single constant answer a replacement for captchas? Why not suggest that to facebook and see how they respond?

if it used one question at random from a few dozen that would be more effective (unless the site was being specifically targeted)

though CAPTCHA isn't effective anyway for all 'bots.

I still disagree. Anything that can be recorded and played back is still insufficient as a 'human test'

all captchas are rubbish...

This is the best I have seen: http://www.dzone.com/links/negative_captcha.html

Why can't they at least be a more readable script? Most of them are impossible and have to be entered several times.

There are existing systems that can read text, so the main trouble with the 'written word' style of captcha (I really should be capitalising that) is that it must be easy to read by humans but hard to read by computer. As computer character recognition gets better it means it must be made harder to read in general and hence harder for people to read. Disappointing but necessary, in my opinion.