Django for social networking
This is not an issue only on Django or python, it's a thing of cloud and software engineering. One server alone may be ok for 10,000 users, given they are not concurrent, also location, are these users in the same city? country? 
I believe Django is very good and I will use it my self in a similar project, my issue is not Django but the IaaS, the infrastructure where I will run this on. 
If you are still worried if Python is the answer then you can research about, Ruby on Rails, and asp .Net, even perl, php, stuff like that. To me, Python is definitely the answer.
I know this is a relatively broad question, but is Django robust enough to build a social network on? I am concerned mainly with performance/speed. For example, for a site with a small user base (<10,000 users), is it possible to create a Django-backed site that would perform at a speed similar to Facebook?
What are its potential weaknesses, and things that need to be focused on in order to make it as fast as possible?