$10 billion divided by 200 million users = $50 per user.
http://www.huffingtonpost.com/2009/0..._n_207792.html
However, I doubt a small social networking site has the same valuation metrics as a Facebook.
If Facebook is worth $50 per user, it doesn't mean your site is worth the same. They have market leadership. But, it provides a range. I am guessing a small site is worth $10-15 per user.