Hello, I know it's not really a programming question but I don't know where to ask it.
Should i use a captcha in my sign up form ?
Facebook, twitter, foursquare, gowalla etc... don't use one (or not a visible one). Is there an invisible catpcha on theses sites ?
Thank you
UPDATE : I have found a nice article about it : http://www.smashingmagazine.com/2008/07/08/web-form-design-patterns-sign-up-forms-part-2/ Thanks for your answers
I don't use a captcha for SingingEels, but I do require a valid email address. Users have to confirm their email address by clicking a link sent with a GUID upon creating a new account.
That validates a real human, and also prevents quick sign-ups for spam comments.
No sense, because sign-up process uses e-mail verification process. It is necessary for a form that does not check user's e-mail (login) existance, for example see this, to avoid spam. Google, for example, displays captcha when you many times input wrong login+password.
Preferably not as they are not user friendly! Most of the CAPTCHA's have been broken anyway ( see PWNtcha )
Security shouldn't be intrusive, and there are far better ways of validating who's browsing your site. Have a look at www.atlbl.com for an API to detect botnets, web spammers, etc etc.