Even more to the point: you can't have a secret that other people know about and aren't completely diligent about protecting. Somebody letting your secret slip

Even more to the point: you can't have a secret that other people know about and aren't completely diligent about protecting. Somebody letting your secret slip offline means one of your social circles will find out, and if you're very, very lucky it will stop there. But chance meetings do take place; your cousin bumps into one of your coworkers in the airport, notices his company shirt, and says "hey, do you know my cousin lotharbot? Did you know he really likes the Teletubbies?" Facebook speeds up this process by increasing meetings between people from your different social circles. But fundamentally, the problem isn't Facebook, the problem is your friends letting your secret slip.