We know Google's app engine is free up to 500 MB of storage and enough CPU and bandwidth to support an efficient community serving around 5 million page views a month. You have a even better news. Vikuit.com provide a software package for free which will provide you with building blocks that enable you to create your own fully-featured vertical social network on it. Therefore, if you are looking for a script to be hosted on Google App engine to make a social network, this may be an ideal option for you. However see pros and cons before you move forward.
Pros and Cons
It is free.
However when I visited their demo I saw lot of spams on it. It means the platform may be vulnerable to the automated spammers.
This is one thing a webmaster seriously counts when moving for a script to be used to develop a web platform. Once you get surrounded by manual spammers you will able to give a fight, but if you a platfrom.