Facebook has announced that it now has 500m active users, just six years after it was launched. The site has become the poster child of social networking on the web. While some others have seen growth, MySpace, Flickr and Bebo appear to have declined in the past year, according to these figures from Nielsen. Interesting international variations are seen, both in the amount of time Facebook users spend on the site each month and in the competing networks' popularity in different countries.