Added an *optional* homework on social networks as promised

It is in the homeworks list--note that this is *OPTIONAL* and will be counted as extra credit if you choose to do it (you will have to view the lectures I had sent earlier to 

make sense of the questions).

The homework will be due by the last day of the class. 

Rao

ps: Note that there will be a *required* homework that is also going to be due on that day. So if you want to do this one, it would be wiser to allocate time early on.