I think this is a very useful thing to know since my friend SJ was asking. Only 4 simple steps.
First click on the word "Applications". This is found at the bottom left of your screen, when you are logged in into Facebook.
When the application options is called up, you will see a list of all the applications that you have approved to run in Facebook. Click on "Edit Applications".
The page will reload. So you have to remember to save or finish playing any games or activities, if not all changes will be discarded.
Again, you should see the list of applications you have approved. Now, the page should display options for you to edit the settings and the profile of the application. Click on the "Profile" of the application you want to see friends who are playing the game but not in your list of game friends yet (eg: neighbors for Farmville or Fishville, sisters for Sorority Life, etc).
In this example, I am using Fishville, as requested by my friend.
The page will reload with the page of the game. Click on the number that appears on the Information box that is on the left (located in the center of the Profile page).
This will bring you to another page that will show you the list of friends you have. You will be able to send them a message to add you in the game.
Hope this will help you. Let me know if you have got other questions! Do add me as "Pek Lyn".
Game tips made easy for you!