After I showed Grumpy Goats to my game design students, we had a discussion about elements of Facebook games, and I showed them World of Secrets.
One might not think a Facebook hidden object game lends itself well to a co-op game for teenagers. (Have I mentioned my amazing gamer/teacher skills.) But we created house rules in which one player was on the computer, and the three others were circling found items on the whiteboard projection of the screen, so when the computer player got stuck, his friends had given him hints already! There was a great deal of screaming and laughing over finding the missing items, and a lot of excitement over completing the scenes.
I really enjoyed this article. When my studio worked on the Wii version of Mortimer Beckett we added modes and mini-games to encourage social gameplay to the terrific hidden-object game. It was a blast to race to find the most objects or cooperatively find objects together.