I recommend validating your market with a newsletter or some other channel that already has social engineering baked in. In the case of Product Hunt & Mattermark, this allowed them to demonstrate value with curation of highly valuable content for a particular group of people. Once you have validated it, creating the application seeded with those users will ensure you have the well in place. Spending time building the app without having the ability to iterate first on the differentiating aspect might be a waste or at least an inefficient use of early resources.