I’ve written an introduction to the Activity Streams standard for IBM DeveloperWorks:
Enter Activity Streams, an evolving standard that extends Atom for expressing social objects. Although it is a young standard, Activity Streams is fast becoming the de facto method for syndicating activity between web applications. For example, MySpace, Facebook, and TypePad all now produce Activity Streams XML feeds. But this technology isn’t just for the consumer web environment. As corporate intranets and internal software become more social, solid business reasons support implementing Activity Streams as a feature. This article describes Activity Streams in detail, considers its potential uses in enterprise environments, and provides some examples for interpreting Activity Streams feeds using PHP.