I have no hearts. Why? They don't show up. I havn't had them since the beginning.
There seems to be a "test group" of users testing a new format of duolingo that doesn't have hearts. If duolingo determines that people learn better using this method, everyone else on duolingo will get it. If duolingo determines that this new format hurts learning, you will be given the format with hearts that everyone else has.
Unfortunately there is no way of knowing how long this test will take, and there is no way to 'opt out' of it.
I've seen a screen shot, and it looks pretty cool, so I am kind of jealous :)