The T-List has a very readable, useful and detailed post, Are Your Facebook Friends Revealing Secrets? The post points out that when you leave a post on a friend’s Facebook wall, their privacy settings (whether they allow their wall posts to be viewed only by friends, or also by friends of friends) will affect how widely your post is viewed, with or without your permission. And there is no easy way to see your friends’ privacy settings, and thus predict how widely your post will be distributed.

The post offers a nice workaround:

Use your own Facebook Status Update to create the post – that will give you access to privacy settings on the post (look for the little lock icon next to the Share button – If there isn’t one, you are writing directly on your friend’s wall – go back to your own wall.)

Type an “@” and you will be able to tag a friend to start the post. Then write the post.

Click the Lock Icon to set the desired privacy level.

Share the post – It will appear on both your wall and your friend’s wall, but your security settings will override the ones governing your friend’s wall

Read the whole post for a great overview of how Facebook handles the privacy settings of wall posts, and how its privacy settings have evolved over time.