I think if you're going to use this "test the waters" method, you owe it to those consumers to let them know that what you're promoting is not an actual service

I think if you're going to use this "test the waters" method, you owe it to those consumers to let them know that what you're promoting is not an actual service but an idea for a service. And that's not how the landing page comes off. Oh, sure, the response rate is probably going to be lower if you're truthful and say, "Hey, I'm doing some market research here. What do you think of this idea?" than if you say, "Hey, I've got this great service that you should sign up for ... which I will actually get around to building if there's enough interest." But at least it's a fair trade. Potential consumers give you feedback about the viability of your idea, and you give them full disclosure about what stage of implementation the idea is really in. Whereas currently, you're getting the feedback you want, but if there's not enough interest expressed for you to invest more time in the idea, then you're leaving high and dry the people who have signed up for the invite, expecting the service to actually exist.