You have to learn the difference between server side and client side. Cheat engine will only work on client side.
Well I believe facebook games are server side right? They are hosted on Zynga's servers and I was able to up the amount of money and products she had. What I would do is load up CheatEngine, Load the FireFox process, test different searches with the value of whatever I was looking for, so say I was looking to increase her gold, I would do my first search on the value, then either spend some or gain some, do another search, etc. Until I found the bytes I was looking for then either freeze it or change it. The only thing is every time you load the game they move so you werent able to save cheat tables like with a regular game, but it definitely worked for online games.