It may be that they don’t have permission to!  If you want your Facebook friends to post to your wall, you may need to update your privacy settings.

a) Go to your Facebook profile page and click on “Options” which is located under the “What’s on your mind? – Share” button then click “Settings”.  Check the box for “Friends may post to my Wall” under the “Stories Posted by Friends” section.

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b)  In the upper right hand-corner of any page on facebook go to Account > Privacy Settings > Customize Settings > Scroll down to Things others share and click “Enable” for Friends can post on my Wall

Whenever you edit your privacy settings it’s a good idea to check how this change may appear to your facebook friends.  Just go to Account > Privacy Settings > Customize Settings > Preview My Profile.  This is how your profile currently appears to most facebook users.  To see how your profile appears to a specific person, look for “Preview how your profile appears to a specific person:” at the top of the screen and type their name in the box.

Keep in mind that nothing on the Internet is ever “really” private – so post responsibly!