Who Can See My Posts on Someone Else's Facebook Wall?
If you are truly concerned about your privacy on Facebook, it is important to consider not only your own profile privacy settings, but also those of your friends. If you are uncertain about whether or not a friend has kept her profile Wall public, post personal information to her wall with caution. You cannot control who sees what you post on a friend's Wall -- only she can.
Whether or not another Facebook user has access to your posts on a friend's Wall is determined by the Wall owner's privacy settings. Each user has complete control over which parts of her profile are public and which are protected, including Wall posts. If your friend has made her Wall posts public it means that anyone in the Facebook community can read what you write on her Wall, regardless of your own Wall's settings.
Wall Owner's Friends
While many users choose to hide their Wall posts from non-friends, most people allow their friends to access their Wall. If are able to see your friend's Wall, then she has made her Wall visible to her friend list and all of her friends are likely able to see your posts. It is possible that your friend has hidden her Wall from select friends, but there is no way for you to access her privacy settings. Always assume that the Wall owner's friends will see your posts.
If the person whose Wall you post on has kept her profile hidden from non-friends, then your own friends will not necessarily be able to see your posts on her Wall. The only people on your friend list that will see your posts are those users who are also friends with the Wall owner. If the Wall owner has kept her profile public, however, then a story about your comment will appear on your profile and all of your friends can link to and view the post.
When you block a user on Facebook, you become invisible to her everywhere on the site, including on others' Walls. Even if you have a mutual friend with the blocked user, she cannot see the posts you create on the mutual friend's Wall. Likewise, you will never have access to the posts a blocked user creates anywhere on Facebook.