There have been a lot of Twitter landmarks along the way stretching back to the 2008 presidential election through to the Arab Spring and the death of Osama Bin Laden.
Twopcharts reported the 500 million figure a few minutes ago and now predicts it will hit 600 million users in around 108 days."
Source: The Wall, 22nd February 2012
Note - These are registered users, and many will be inactive or automated bots. Nut no one is doubting that Twitter is popular these days!