Windows Live activities on the “People Hub” of Windows Phone 7 are powered by Activity Streams. Also, while the screenshot shows only Windows Live and Facebook, if you connect other services to Windows Live like LinkedIn, YouTube, Flickr, WordPress, and ~70 more, you and your friends will also see these in the People Hub experience on Windows Phone 7.
For e.g. what a Windows Live Activity Streams feed looks like, browse your Live Profile, then click orange RSS/Atom button in your web browser to see a public version of that user’s activity stream. You’ll see a URL like: http://api.live.net/Users(-5141839532282463176)/Activities?$format=atom10&mkt=en-US and if you View | Source, you’ll see the Activity Streams feed, just like what’s used by Win Phone 7:
More Info: Activity Streams