A secure democratic Net is a fundamental right

If you accidentally did that, then change you facebook password immediately. If it has already been changed by the rogue malware (there have been reports to this effect), then you can use the lost password link to reset your facebook password

http://www.facebook.com/security

source http://anshprat.wordpress.com

and you should change all the passwords in the other portals and services where you use the same passwords or a small variation on them

and just the same as with online banking, never click on a link, close all your other webservices (chat, cam, P2P, ....) and open a new browser as the only one and type the domainname with your own hands (no links or other stuff like that)

and patch your browser and pc always