Since they opt out the Privacy setting to hide the activity stream every activity you make in Facebook shows up in your friends stream . When ever you like a page join a Group comment a status it will popup as activity in your friends stream . As many like to comment and be much more active with there friends without bothering other friends with your activity you can switch it off by turning your wall to Only ME.
PRIVACY – CUSTOM SETTINGS -POST BY ME – ONLY ME
What does that it will Block your Wall for every buddy and hide any post you make on your wall or on other Walls Photos Videos its finally hide every activity you make .Except to that person the wall belongs to .