Actually, you can absolutely delete all of your facebook posts. It's pretty difficult and is not for internet newbies.
Basically, you download IMacros--which if you aren't familiar with them, they are capable of doing robotic tasks (for example, deleting individual posts).
You download it, create an .iim (using "play" and "stop") and then edit it to include this code:
(leave the first version line)
TAB T=1
SET !ERRORIGNORE YES
SET !TIMEOUT 100
SET !DIALOGMANAGER NO
TAG POS=1 TYPE=SPAN ATTR=TXT:Remove
TAG POS=1 TYPE=INPUT:BUTTON ATTR=NAME:delete_story&&VALUE:Delete
WAIT SECONDS=3
TAG POS=1 TYPE=SPAN ATTR=TXT:Remove
TAG POS=1 TYPE=INPUT:BUTTON ATTR=NAME:delete_story&&VALUE:Delete
WAIT SECONDS=3
TAG POS=1 TYPE=SPAN ATTR=TXT:Remove
TAG POS=1 TYPE=INPUT:BUTTON ATTR=NAME:delete_story&&VALUE:Delete
WAIT SECONDS=3
TAG POS=1 TYPE=SPAN ATTR=TXT:Remove
TAG POS=1 TYPE=INPUT:BUTTON ATTR=NAME:delete_story&&VALUE:Delete
WAIT SECONDS=3
TAG POS=1 TYPE=SPAN ATTR=TXT:Remove
TAG POS=1 TYPE=INPUT:BUTTON ATTR=NAME:delete_story&&VALUE:Delete
WAIT SECONDS=3
REFRESH
Then go to your facebook page and play the iim. And it will delete your posts.
I know--that isn't as detailed as it could be--so I found a tutorial for you.
http://www.cynthiaperla.com/facebook/460/how-to-delete-all-groups-and-wall-posts-from-facebook-using-imacros.html
You're welcome. You can thank me by making me best answer.