Sony announced today that the Playstation Network exceeded 20 million registered users world-wide this month.
The registered accounts on PlayStation Network reached 5 million in 1 year and 2 months, 10 million in 1 year and 8 months, and exceeded 20 million in only 2 years and 3 months from the service launch on November 11, 2006.

Sony says that the new 3D world PlayStation Home launched in December 2008 has 4 million users. Life with PlayStation has now over 2.6 million users.

The PlayStation Store offers more than 14,500 diverse digital content, ranging from exclusive on-line games, downloadable version of UMD titles, game demos and items, and titles from “Game Archive,” through which legendary and popular titles from the original PlayStation are made playable on PS3 and PSP, to more than 5,900 movies and TV episodes that are available through Video Delivery Service that started in the United States in July 2008.

To date, more than 380 million pieces of content have been downloaded, with total sales exceeding 180 million US dollars.

Sony currently has 21.3 million PS3 units as of December 2008 shipped and 50 million Sony PSP units.
This means not even a 3rd of the Sony consoles are registered on the Playstation Network yet. It is a bit surprising how "offline" gamers still are, especially as the Playstation Network is free of charge.
Via this Sony announcement.