-usertesting.com - you can get the profile of customer you're looking for go through the entire experience without needing to deliver (if shipping products). Their experience with the landing page is recorded in video and you also get text answers.
-reddit/hn - depending on the market there might be a good match for a subreddit and full/no match with the HN crowd.
-craigslist/kijiji - this is the blunt definition of concierge MVP for a new product :)
I will advise against FB ads. Hyper targeting is great but those ads are invisible to people.