This method publishes a post into the stream on the Wall of a friend or a Facebook Page, group, or event connected to the current session or specified user (but not to an application profile page). By default, this call publishes to the current session user's Wall, but if you specify a user ID, Facebook Page ID, group ID, or event ID as the target_id, then the post appears on the Wall of the target, and not the user posting the item.

==>target_id

string

The ID of the user, Page, group, or event where you are publishing the content. If you specify a target_id, the post appears on the Wall of the target profile, Page, group, or event, not on the Wall of the user who published the post. This mimics the action of posting on a friend's Wall on Facebook itself.
Note: If you specify a Page ID as the uid, you cannot specify a target_id. Pages cannot write on other users' Walls.
Note: You cannot publish to an application profile page's Wall.

http://wiki.developers.facebook.com/index.php/Stream.publish