You know what's incredibly creepy? 3000 faceless people in Palo Alto know more about your distant friends than you will ever know.
Depending on how much time you spend on the site, they probably also have profiled a sizable portion of your thoughts simply by tracking clicks and pageviews.
Your chemistry class example is nonsensical. In class, if there is an opportunity to explore a few things and a mess is made, maybe you would not be blamed. That's usually not how labs are run--you follow a procedure and mixing chemicals with no forethought is a huge safety hazard to everybody in the lab. Neither the "real world" nor the Internet is a place with a mutual agreement between all participants to experiment with each other's property.
Maybe a better example would be going into your neighbor's backyard and testing how readily his shrubbery lights on fire. Oops, it's burning! Tell him to "fix the bug" and move on.
Depending on how much time you spend on the site, they probably also have profiled a sizable portion of your thoughts simply by tracking clicks and pageviews.