I just learn about tabnabbing.
Tabnabbing is a computer exploit and phishing attack, which persuades users to submit their login details and passwords to popular websites by impersonating those sites and convincing the user that the site is genuine. [...] The exploit employs scripts to rewrite a page of average interest with an impersonation of a well-known website, when left unattended for some time.
It is mentionned that you can detect to which website a user is currently logged in.
How would you proceed to detect is logged in Facebook from another site for example?