There is a post going around to scare people. There is numerous different variations of it. Here is one of the posts going around

HEADS UP, EVERYONE!!!! Was just informed that in addition to Porno Videos, and photos, there is a new hack on Facebook. It appears comments coming from you includes dirty, hurtful phrases. You cannot see it, but your friends do. This situation can create many misunderstandings. I would like to say to all my contacts that if something shocking appears, it absolutely is NOT coming from me and I would be grateful if you let me know.
Thank you very much!

This is not a valid threat. If someone posts something on your wall, you will see it. They cannot hide it from you. If your friends can see it, so can you.

There is only one way the hacker can post as you without you knowing about it. That is to create another account and make it look like your account. They do this by doing the following (for example purposes, I am using Jane Doe) Jane Doe has a Facebook account. On that account, it has a picture of Jane Doe riding a horse as her profile picture. She has three pictures in photos of her fishing. Her email address is Jane.Doe@example.com.

A hacker does the following 

• Create another account with the name Jane Doe

• Since the account is based off the email, the hacker will use another email address that he or she stole.

• The hacker will visit the valid Facebook page and download Jane Doe's profile pic and pictures in her album

• The hacker will then post those pictures on the fake profile. The hacker will also make the picture of her riding a horse as the profile pic

• The hacker will visit the her friend's Facebook pages and send them a request to become friends

• Some friends will be suspicious of this and will notify Jane Doe

• Others will think she either accidentally removed them as friend and will accept the friendship.

Now the hacker can post bad stuff about Jane Doe. Those friends that accepted the fake account will see the bad posts and the valid Jane Doe will not know about it, but it is not her account. It is a duplicate account.

There is some things you can do to protect your photos from being stolen but you cannot prevent a duplicate account from being created. If you determined a duplicate account has been created you can report it to Facebook.

If you want to protect your pictures, you have to tighten the security of your account. However by doing that, not everyone will see your photos. You will need to decide what is more important. Go to Privacy for more information.

Whenever you get a friends request, always go to your friends list and see if that person already exists in your friends list. Some people do have more than one account but if you do get a request from someone already on your list, contact that person to see if the second account is valid before accepting it.