Instead of building the product first, try and see if you could use some existing service to test our your market. For e.g. you could start a Facebook Group, or a mailing list to run things manually for a while. Get the user engagement, get a lot of feedback. And once you have enough material to work on, build the product. In other words, get to market without building a product if you can. It's always easier to get some traction on existing (social networking and other) sites than a new standalone site.