And, these are active users we're talking about. How they determine this number at Facebook is unknown to me, but it probably means that users who've just created an account which sits idle for a long period of time aren't included. This is huge; Facebook is growing incredibly fast, and at this pace it will will reach 300 million users by the end of 2009.
Also worth noting: Facebook was once a US-only service, but now it has users from over 170 countries. I can personally attest to Facebook's growth in Croatia, where recent numbers suggest a growth of 100% over the last six months of 2008, with over 400,000 users. If this example is any indication, Facebook has no problems expanding internationally, in fact, it might even be growing faster than in the US.