Actually, looking at the top five countries in the geo breakdown, Facebook is dropping in a lot of places in the world. Not sure what's going on there.
Comparing the US and worldwide graphs, they appear quite similar to me. The world graph has a strong rise mid-2010, where it raises above the US line for the first time. Then, they both level out with some jig-jogs shortly before the start of 2011.
Actually, looking at the top five countries in the geo breakdown, Facebook is dropping in a lot of places in the world. Not sure what's going on there.