If you have checked in with Foursquare in San Francisco in the last three weeks, Jesper Andersen probably knows where and when -- even if you've set your check-ins to be published to friends only.
Andersen, a coder who recently built a service called Avoidr that helps you avoid social network "friends" you don't really like, figured out that Foursquare had a privacy leak because of how it published user check-ins on web pages for each location.
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