Hey,
The done class is applied based on browser cookies. If you have a post with the id of 82, for example, after you click 'like', your browser has a cookie added called like_82.
If you have this cookie, the post with the ID 82 will automatically have the 'done' class applied to it when it's rendered.
I wonder if something weird is happening with your cookies. If you're using Chrome, you can go into your web inspector -> resources -> cookies and take a look at what cookies you have and make sure they're being applied correctly. It's hard to say for sure why it isn't working, but that's my best guess!