It blows my mind that linkedin exists. I believe its Zero to One that talks about how Reid realized rather than try to replace recruiters he made a tool to help them do their job.
Non recruiter/sales people use it as some type on online resume and look at how great I am page while recruiters get access to everyone they could ever want.
Having trouble getting a job? Just pay us $20/month and more people will see your res... er, profile! Too bad visibility is not the solution those people need. Just one that is trivially implemented and easy to sell.