I've had to answer the social CAPTCHA myself, when logging in from a different country while on vacation. Details:

You are shown two pictures of the same friend, with the tag of him/her showing (i.e. a square around them, like what you get when you hover over their name in a tagged photo). You are then asked to say which friend of yours it is, choosing from a list of around 7 names.

You can skip as many of these questions as you like, in case you've got pictures where the tag isn't very good (e.g. someone tagged a comic or something instead of an actual photo). But every question you answer has to be correct.