Many web designers are striving to create a community around their websites.  A community is extremely helpful when it comes to allow members to create more contents for the website.  Some of the popular choices that allow us to create a social network like facebook or my space are as follows.
1.Ning
2.Socialgo
3.Socialengine
4.Jomsocial

While Ning and Socialgo are companies that host the social network for your, Socialengine and Jomsocial are seperate scripts that could be hosted anywhere.  Ning and Socialgo try to offer an all in one package that give you peace of mind.  The packages are simple and easy to use, but they do not give you full power to express your imagination.  For advance website developers, these services limit or hinder them tweaking or modifying their websites to the way they would want it.  Once, one has started using these services he or she is somewhat forced to continue using their services (hosting).  The marketing stratagy behind thes company is to sell the script and hosting plan at the same time.

On the other hand, Socialengine and Jomsocial give you the right to own the licence .  You are not obligated to hosting with their company; you have the freedom to take the script anywhere and host it.  The drawback of using these scripts is the learning curve.  It is not as simple as plug and play.  You have to know how to install the script, configure it, and monitor it. 

If you are someone that likes to have a plug and play system then you should go with Ning or Socialgo.  This option give you less headach, but limit your freedom.  If you like to own your own script and the freedom to tweak your social network in anyway that you desire then go with Socialengine or Jomsocial.  This option give you the rights and the freedom to follow your imagination.