If you want honest comments rather than spam comments, I have used G+ comments (there is a plugin for this) and I know there are other services like it using FB, Twitter, etc. If you're not a spammer, you can always go see and respond to these comments, and interact with the folks in very natural ways. Who needs mass-marketing like email anyway? No one reads it... Be personal, and up-front with people about what you are pitching, and you'll probably make a sale.