Recruiters and third parties have already signed up. Everyone uses it. That's the point.
If that is the case, then where is the revenue growth going to come from?
New market segments.
LinkedIn is incredibly successful in IT related markets, mostly in the US.
It's like when Amazon started, it was very successful selling to people involved with the internet in some professional capacity.
LinkedIn earned their 100 million users by providing a professional networking platform where the users could choose who they network with. When the userbase has less of a choice regarding who contacts them, they'll abandon linkedIn as fast as a teenager terminating his MySpace account.