Best Solution for Social Backend
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What's the simplest way to deploy a back-end server with social features? The basic features would be

 - User Profiles
 - Friends
 - Activity Streams
 - Notifications

Most of the off-the-shelf solutions I find are targeted at social networking "websites" whereas I just need a back-service which provides APIs to be integrated into mobile applications.

I am checking out Open Social but doesn't seem to be as 'out-of-box' as I thought it would be.

What else is out there?