Finally, Facebook Messenger for iOS has stickers. Android users already have these stickers since the end-April period. Facebook has finally joined some of the IP messaging companies (KaokaTalk, LINE and etc) to provide stickers.
With the latest update of the Facebook Messenger, you should be able to see a smiley sign besides the area where you key in the message. Click on it to access these stickers. Click on the basket to get more stickers.
From what I see, iOS has more stickers than Android at this moment. So, start bombarding your friends with stickers now.