It's interesting that 3 bonus skills are available in the iPhone app store, but only 2 bonus skills available in the browser store.
Two bonus skills are available: idioms and flirting. This all depends on where you are in the tree. You do have to learn certain skills before being able to unlock them. Christmas was a seasonal bonus skill that is no longer available. It is a little confusing because we make it appear currently as though there is a third. We'll work on that!