This is insidious and it's happening more and more often. There are two good lines of defense:
- If you receive a Friend request from someone who is already your Friend, DO NOT ACCEPT THE REQUEST! Please contact your Friend directly (message, phone call, e-mail), and ask him if he sent you another request. Frankly, I don't know how to send a Friend request to someone who's already my Friend... so this shouldn't be possible! Confirm that he has NOT sent you a Friend Request, and let him know that he's been hacked.
- Change your password periodically. I know. It's inconvenient. It's a pain. It's not fair. But this is the best way to protect yourself against hackers.
Facebook is a wonderful tool for keeping in touch with friends all over the world, but it is not great at security or privacy. NEVER post your Social Security number or Credit Card Number or any other information on Facebook. (Just as you should never write these numbers in an e-mail.)
Enjoy Facebook, and protect yourself at the same time!