Open the game page from Facebook and click the 'Report/Contact' link from the bottom right corner of the page. From the opening popup choose 'I'm having a payments issue'.
If you made an order but did not receive it, choose the option 'I didn't receive my in-game item or virtual currency'.
If someone else made a purchase on your account without your permission, choose the option 'There was an issue with a payment in Angry Birds Friends'. After you have received a refund you should consider removing all used payment options from your Payment Settings and only add them when you are personally making a purchase.