Note: Outside users (people outside your company) have profiles, but they don't have walls. Outside users' names appear in purple text, and they have icons () after their names. If you want to post something to an outside user, you can do so through a one-on-one conversation. For more information, see How do I start a one-on-one Conversation with someone?
When you post on someone's wall, the post is visible to other users (unlike a one-on-one Conversation).
To post a message on someone's wall:
You must be following the person to post to their wall.
If you don't see the text box at the top of a person's wall, you aren't following the person or the person doesn't allow other people to post to his or her wall.
To learn how to control who can post to your wall, see How do I change my preferences?