This is a demonstration of how a website can detect which Social Networks a user is logged into when they visit. In my tests it seems to work in all the major browsers (Firefox, Chrome, IE 7+, Safari and Opera).
For details please see my post on how to detect which social networks your visitors are logged into.
If you want to prevent sites from being able to detect this then for Firefox you can try RequestPolicy or NoScript. For Chrome you can give ScriptNo a shot. On IE you can try giving Firefox or Chrome a try. ;)