This is BIG. Facebook officially opens up the news stream.

Facebook have just announced news of a dramatic change to Facebook’s core offering, the news stream.

Via the new Open Stream API, developers can create applications to read and interact with the stream, as you would do on Facebook.com.

“With these new methods, you can access the stream on behalf of a user and then filter, remix, and display the stream back to that user however you choose, wherever you choose, in the manner most relevant for the user experience. Other new API methods will allow users to both publish into the stream and to add comments and ‘likes’ to posts in the stream.”

This means desktop applications like Seesmic Desktop (who already have it (partly) integrated) and web based apps like Friendfeed can interact directly with the Facebook news stream (via a single API call apparently).