Django can certainly be used to build a social network, It offers great features for performance enhancements like caching. See this post on scaling.
The main bottleneck will come with how you design your models. In my experience, creating deep nested foreign links and many joins(manytomany relations) slows up when you are running complex queries. You should try listfields for such cases. You can also investigate the key/value pair Google uses on its big table in appengine, it scales more than relation databases.
You should also page items conviently, you may want to use ajax to still keep the user experience and prevent users from loading pages just to see more posts.
