Facebook has opened its doors to outside developers by signing up 65 new partners.

The deal effectively turns the social networking service from a closed site into a platform on which third-party web applications can run.

The 65 firms announced today include big names such as Microsoft, Amazon and Photobucket, as well as instant messaging site Twitter and VoIP service Jajah. 

"Until now, social networks have been closed platforms but today, we are going to end that," Facebook chief executive Mark Zuckerberg told an audience in San Francisco.

In one of the first partnerships to be completed, Amazon unveiled a Book Reviews application that runs on Facebook.

The add-on allows users to share thoughts on books with other people in their circle of friends, as well as selling the books from the online retailer.

Meanwhile, Facebook unveiled an application developed by its own engineers that allows users in a group to share live video from webcams or mobile phones.

Videos shot using a cameraphone can be instantly uploaded onto the site and added to the user's Facebook profile.

Facebook has also created its own variant of the HTML programming language that can be used by members to create pages with adverts or other tools.

Facebook currently has 24 million active users, making it the second largest social networking site behind News Corp's MySpace.