In the last few years, developing a new social network was a very cool idea, however, I believe nowadays developing such a system might be quite silly if there is no justification or innovation behind it.
For instance, back to 2004, when Mark Zuckerberg started building Facebook, his idea was quite unique and awesome. However, now creating a conventional new social network is as easy as installing WordPress with variety of the great and free social network CMS availability.
Hence, I personally do not think there is a need to create a social network from scratch anymore.
Instead, new ideas can be implemented directly in the current social networks (such as Twitter, Facebook) or in the worst case one of the ready made social networks CMS (Elgg, Pligg, BuddyPress, etc. ) which has the basic features of social network platform can be used as the foundation of your framework/idea/work.
Using social network CMS can be beneficial in various aspects. First of all, you do not need to spend time, money and resources to build a foundation of a social network from scratch. This wastes lots of time and it is also very cumbersome.
Secondly, social network CMS are developed by a community which is well architecture, documented and tested, hence, no need to go through the painful process of troubleshooting and debugging in order to create a clone social network. In addition to that, any identified bugs can be reported and fixed by the community for free mostly.
Lastly, you have more time and less stress which gives you high chance to concentrate on your idea/framework more to enhance and deliver better product and avoid doing duplicate job and reinvent the wheel.
In the following links you can find bunch of social network CMS,