Is ~ $10/user a good deal? I think so. That should be easy to make back.
Plus, as Rod said they would be acquiring a socially connected, thriving team and platform.
I've heard it's not always easy to start a business in China. I've heard it compared to the early days of the industrial revolution here. I've had a few friends from countries in Asia observe that, it's not only about how skilled you are, but how connected you are and what those connections allow you to do.
Only 14% of China is online - a number that is already larger than the number of connected Americans and growing rapidly. It is essential that Internet companies tap into this massive market.
I give kudos Zhanzuo for doing so amazingly well. And think it would be a smart move for Facebook.