it's possible only in wi-fi networks and you can stop your account getting hacked.

STEP 1:(how to hack)

1.Download firesheep Firefox extension.This is a freeware extension for firefox browser. 

 

2.  Once installed it will open a sidebar window into your firefox browser.

 

3.Now it will show all the people who are connected to unsecured wifi network.Once they login into your facebook or twitter account you will get a notification and with a single click you can login into their account.

 

this whole trick called cookie hijacking..

 

STEP 2:(how to prevent) 

simply doing https:// you can prevent your facebook account from hacked. 

 

click link below to see how to prevent it.. 

 

 http://thingss-to-know.blogspot.com/2012/01/how-to-stop-facebook-account-hacking.html