My guess is they haven't taken off in NA/Europe as other parts of the world (most notably Asia Pacific). But they are definitely available. They look like very large emoticons.
They just became available recently.
Some of my friends use stickers all the time- those of my friends who use facebook messaging a lot. You can even buy expansion sets. I should note these people are all 30-something (you can tell by the fact that they are still using facebook messaging)
It's just rolling in. For my account it got activated today. It basically allows you to post stickers from Messenger as comments.