I agree with the previous advice you have been given in this thread.
#1 -- Decide on a price you can live with, but don`t reveal it
#2 -- Find out what the companies are seeking. What you are looking for here is future value, not how much it will cost to replicate or monitor the site. Your site has social networking value. What is the value to them? Get them to name it.
#3 -- Remember a key rule of negotiation. Silence is your friend. To tell too much too soon. Find out where they are coming from first and negotiate from that perspective.