Rilakkuma US - On Facebook Messenger you can now use these free Rilakkuma stickers to chat with friends! Open Messenger, click the smiley face, then the + plus sign on the right!
Messenger - Creatively express yourself with Facebook Stickers! Here's how to access and send a sticker: tap on the smiley face, which will bring up your sticker options. Tap "+" to access
Usagyuuun and Nekogyuuun - Facebook Sticker Repository
How to use Facebook stickers in iMessage conversations