Hi! This sounds like a similar question I just answered! Lol. Check out my responses and look for the latest ones, and read on that a bit.
The websites regarding this are Thumbtack and Believe it or not, Clarity. I personally use this platform for marketing purposes, to get my name out there a bit more for my own firm, we do online marketing, web and app development and have actually acquired many leads on projects through here.
There are however many websites that offer a social CMS platform where freelancers can offer their skills like fiverr.com too or elance.com too i think offers this.
The revenue model seems to always be consistent to the finders fee- they process the payment, provide you with a profile website and marketing tools and take their share off each gig. Uber and Homejoy do the same thing... Their value offerings are different but the model doesn't get reinvented.