"FriendFeed aggregates all of your activity from the sites you choose using web crawling technologies similar to those used by search engines. For most sites, all you need to provide FriendFeed is your username, and the FriendFeed crawler will automatically find and broadcast all of the actions you perform on that site. For other sites, or for services that contain private data and require special forms of authentication, FriendFeed takes advantage of the APIs provided by those sites to collect your activity.

From their FAQ: http://friendfeed.com/about/faq#how