Friend Stickers are used to upgrade Landmarks. You can get Friend Stickers by completing mini-events when visiting another player's mansion. Simply tap on the mini-event bubbles when they pop up and get rewarded with Friend Stickers! Keep an eye out for those bubbles!

You can also get Friend Stickers from visitors on the Message Board.  

Note: When all Landmarks are upgraded to level 6, or you have enough Friends Stickers to upgrade all Landmarks to level 6, mini-events will be temporarily disabled. However, you won't see mini-event bubbles popping up again until a new Landmark appears.