There was a trick in "Four hour work week" - I'm going from memory, might not be accurate - where Ferriss would place eBay listings for nonexistent products and

There was a trick in "Four hour work week" - I'm going from memory, might not be accurate - where Ferriss would place eBay listings for nonexistent products and then cancel the auction before it completed; any bidding would indicate real customer interest. You could do something similar with facebook ads. Make ads for the finished product, if people click take them to a "Register for more info" page. Now you have some email addresses of interested people. This only measures superficial interest in a concept of course; it doesn't give feedback on your actual product.