How do you react when you find out your friends are talking about you behind your back?

Depends upon the friend, how close we are, and how I feel about them. If it's an acquaintance type friend, I generally don't interact with them anymore. There's no point in creating an argument. They've shown me who they are and they aren't people that are worth my time. If it's a close friend, it's a huge rarity for that to happen, and it does get discussed. In the end, I feel a trust has been violated and I wind up keeping them at arms length, lessening my interactions with them. I believe it shows me that they don't feel about me the way I feel about them.