I have mentioned my love of casual games before, especially my addiction to all forms of Bejeweled. I also happen to love hidden picture games, and while I have never bought one, I download the free demos all the time. Just like it has Bejeweled Blitz, Facebook now has hidden picture games. There are two. Only Gardens of Time is worth playing.
It combines game play (hidden pictures), building your own garden, a need for friends, progression, and achievements- pretty much everything you need for a successful Facebook game. Definitely something to check out if you like hidden picture games.