Yup. I`ve implemented Tweet, Leaderboards and facebook in cocoonjs using Clay.io plugins.
but I do noticed that sometimes that the api is a bit slow. Meaning that the games is already started few seconds but the connection to Clay.io or "Clay.io is ready" status is not ready.
If you tried my game, you will noticed that sometimes the Clay.io buttons (FB,Tweet and leaderboard) might not be visible.