So, where did the extra 51 million "unique visitors" come from?

Surely, some of them were users without a Facebook profile visiting via search results and other links, but 51 million of them? One third of all Facebook visitors?

This seems unlikely. Perhaps ComScore is double-counting users somewhere -- and the many users who check their profiles at work and at home are obvious candidates for being counted twice.

Third-party traffic tracking services have always been as much art as science, attempting to track users utilizing statistical panels and educated guesstimates to determine traffic patterns. However, it seems unlikely that ComScore would overestimate by 50% -- and so the question remains. How many people are really visiting Facebook?