Not to over-simplify things, but think “Facebook in a Box.”
Groups, Wire posts (think Facebook Wall), activity streams, friend lists and forums are all a part of the platform. These all build from the user system native to WordPress and simply add functions to allow users to interact with each other. Use as many or as few of the capabilities as you like.
The site also integrates with the Windows Live, allowing Live users to authenticate instantly — something I believe is a first for a non-Microsoft site.
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