How do you value LinekdIn

I have been told by many who have used or invited to join LinkedIn.com that the professional network is some what less useful. Unlike Facebook or other social networks you can freely communicate with others within the community by sending "be friends?" messages to them. There is NO NEED to know the other social network resident's email address. LinkedIn does not support that. LinkedIn insists to collect from $25 to $500 per month if a LinkedIn resident want to send a linking invitation to a resident UNLESS he/she already has that person's email. No wonder when a few friends knew that I received an invitation message to join LinkedIn, they warned me that LinkedIn might not be useful. I soon found out why my friends made such a warning. If Facebook is valued at $5B, Twitter at $1B, what do you think LinkedIn could be valued at?