I would do the following:
If you still have to build your (final) product, I guess what you want to do is a concept test.
One very cheap way to do that is to create a website (or just a web page) where the concept is presented and where you promise a discount for the product, when it will be ready, for users who register (by leaving their email). You then create an adwords campaign with relevant keywords and see the results, if possible testing different concepts. This way you not only test the concept itself, but also your communication campaign, which is also very useful.
If you want specific feedback you can include a survey on the page or website, at least for those who want the discount.
User feedback and involvement is necessary and very important for a company. The more the users (customers) are involved in business the more loyal they'll be.
When customer contributes to your business free (you do not give any reward like discounts) they simple ask themselves why should i contribute my free time writing reviews on this web sites? If you can explain this value to your customers then they will put feedback, but if the value is still not clean for user then do not expect any feedback.
In product development many companies exploited the benefits of crowdsourcing. Google it there is plenty of stories on this.
But for existing product or service you better to offer some discounts or reward points for participation. For example most dvd, book stores reward up to 10 cents for a book or movie review. Simple think for 1000 reviews you only loose 300 dollars (3 cents for a review) but gain a lot review base and more interesting web store to visit. It really matters. Some businesses offer cheaper book prices than Amazon but no one cares because Amazon got a lot of reviews, which are decision support for customer .And also do not forget to quickly respond to feedbacks. It is another powerful method of showing the value of involvement to your customers.
Indeed 10-20% of users write reviews but >80% just use them. But anyways try to involve your users to actively participate in business.
It is actually the same in social media marketing. Small businesses open a facebook page and simple think users will everyday visit their page, this visit will increase day by day and the business will get loyal customers. You have to give your facebook user value, show and make it easily understandable the value of time they spent on you fb page.
Hope it helps a bit good luck:)