I can't figure out how to do this, and the question isn't answered.
Hover mouse over friending activity to see the small 'x'. Click on it to view the options seen in the image. Now to hide a single activity choose the first option - Hide this recent activity story from Timeline - or to hide all friending activity hit the second option - Hide Similar Activity from Timeline.
If you simply want to hide the activity from your friends, while keeping it visible to yourself, do what the first comment instructed. Go to https://facebook.com/friends. Change friends privacy setting to "Only Me".