FB is the largest single referrer to our site. Using google analytics tracking, I can see that inbounds from facebook are directly linked to tens of thousands of dollars in sales. I have had terrible luck with facebook advertising, but we get great results when people share their bike rides on our site using facebook. Their friends who are probably bicyclists themselves see the link and come through our funnel.

You need both a good funnel, and a message that provides value to a user. It has been crudely expressed as "do you get your user laid", less crudely as "make your user look cool". If you do this, then they'll share your service to all their friends for free. In our case, someone is proud to ride 100 miles, and they'd like to show it off. Our tool let's them do it, so they paste it around on facebook. It's all about your value prop and how someone feels sharing that with others.