Why Use Online Business Networking Sites?

I’ve used online networking site Linked In, Ryze, Company of Friends for various reasons over the years. The only one I found really useful was Fast Company Magazine’s Company of Friends. I’ve met MANY online friends that have helped my business grow and that I have been able to help.
On Dave Taylor’s Q&A web site someone asked Dave, I think I understand the value of networking as well as the next businessman, but for the life of me, I don’t really see what sites like LinkedIn, Ryze and Ecademy can do for me. What’s the point of these sites other than just as some sort of digital popularity contest?
Dave has a great answer on his web site by his good friend and colleague Liz Ryan, head of the women’s power networking group WorldWIT, Women in Technology.