A recent project of mine has been ContribKanban.com for the Drupal community. The name speaks for itself - it provides a way for contributors to work on Drupal.org project issues queues through a kanban board. This app has two purposes: visualize the Drupal.org project issue queues in a useful fashion, and provide an experiment that tries to fully utilize Drupal.org APIs.
I've been hard at work in the labs revamping the Facebook Album plugin for WordPress. Very soon you will be able to use a Facebook application to authenticate with the Facebook API.
Google+ has released a rough outline on their API for accessing data. The Google+ API only allows for read-access to data, but this will still allow you to integrate a visitors social features around Google+.