According to Hitwise, nearly 1 in 4 page views in the US took place on Facebook.com for the week ending November 13, 2010. The market share of page views for Facebook was 24.27% last week, 3.8x the volume of the 2nd ranked website YouTube with 6.39%. This is s big jump from just March when the figure was 1/10 page views or 10.28%.
That growth is pretty phenomenal, and looks set to continue.
In addition to the page views on Facebook.com, there are also the users of Facebook off of the main site, in functions like Facebook Open Graph. Many people have Facebook interactions on third-party sites.
The viral loop created by Facebook is so hugely infectious it's taking over the whole web, and the brutal efficiency Facebook have applied to ensuring that growth continues is testament to their management strength. Its gone wrong for many other networks much earlier than this.
When Facebook reaches 1bn users - which I reckon will happen towards mid-2012 - then strange things will start to happen to the rest of the web, and web giants like Google will need to adjust to a new reality.