Activity messages allow you to follow all the activity on the Web site.When users post content,
modify status messages, become friends, and so on, the activity
displays in your activity list. One of the activity message features is
the ability to reply to users' status messages.
When you view the list of status messages in your activity list, you will see a Reply functionality.
Click Reply to post a response to the user's status message, then click Post.
The user will see your message in his/her activity list, along with a text box to which he/she can respond.