Duplicating what businesses such as Facebook and Digg have done is not an easy task. Those sites did not get to where they are overnight. Besides, the market conditions are now much different so the same ideas may not be as unique and successful as they could have been years ago. Still, niche social networks are very much in demand. If you are passionate about a narrow topic, there is no reason not to start a community around it. PlusOne for WordPress allows you to do just that.
PlusOne turns your website into a social community quickly. Your visitors can sign up for accounts and start adding their articles to your website. Others can vote on their stories if they determine them to be of some value. They do get a +1 button to vote for their stories but their votes are not final by any means. Voters can take their votes back too.
This theme comes with 5 color schemes. It has a powerful back-end that allows you to make your site SEO friendly, improve its navigation, add scripts, and include Facebook/Tweet buttons on it.
The submit and pages are actually pretty clean. You get 7 page templates to choose from. The short-codes included can make your website more dynamic.
You are still going to need a few plugins and minor modifications to make this perfect. I strongly suggest strengthening your back-end against spam attacks (considering that you will be allowing user registrations). Adding more social buttons to your site would not be a bad idea either. This theme can save you a lot of time setting up your social network though.