The graph is US traffic only and they peaked at 16.6% of the entire US population visiting the site once a month. Google called this the "law of large numbers" in 2006 when they predicted a slowing in their growth and Microsoft did the same a long time ago when they hit saturation.

The drop is only during the last 6 months, is less than 10% and could be a variety of factors including a stats gathering quirk, recession, etc.