Facebook is asking a select number of users if some of their friends use their real name. This is done in order to test one of Facebook's latest experiments. Facebook faces a problem among people who use pseudonyms instead of their real names.
Facebook insists that all its users sign-up with their real name. The popup that asks you to snitch on your friends is titled: "Help Us Make Facebook Better". Here you are presented with one of your friends' name and picture and a 4 variant poll question. The question is: "Is this your friend's real name?" and you can choose between either: "Yes", "No", "I don't know him/her" or "I don't want to answer".
Once the form is submitted Facebook assures you the data you sent is anonymous and that your friend's account will not be in any danger if you rat on him. Facebook explains why they don't allow fake names on their Name policy page.
The limited test done by Facebook is meant to identify some users that use handles or nicknames instead of their real name. The company will most likely use the data gathered from the test to enforce measures against such cases. If the experiment will be considered a success Facebook might require that we all snitch on our friends.