What's the right way to authenticate survey takers?
javert: Send each participant an email which contains a link to the survey. The link contains a hash key. Each hash key is only valid once. So, each link is only valid once. If you're really worried, also have people do a captcha before they take the survey.I'm not sure if this works with your setup, but presumably, if you have access to peoples' Facebook information, you have access to their emails.