How to validate startup ideas? No really how?

I have read around the subject fairly extensively, lean startup, running lean, the mom test ect. Set up a landing page, do not focus on the solution, only focus on the problem. Find out where your audience hangs out online, ask questions like "Has anyone experienced xyz problem or is it just me ......", "does anyone here use any software for abc ......." Try and redirect them to your email list. Build up a list of 100 interested customers..... launched product, while still building your list and refining the product. Still do interviews whilst building the product, you have to nail the product/solution, problem/market fit. However....... It never really works for me. I usually get banned or my post removed. No self-promotion allowed on Reddit, Facebook groups, here in this discussion group ect. And you can't start the post with.... I have an app idea.....Or the users catch on, sounds like you are launching an app? I have tried paid advertising using Facebook lead gen. This just burns through cash. If I go direct add 100 people on LinkedIn I am lucky if I get 2 or 3 feedback, never mind scaling to 100s of pre-sale signups. How do you actually validate and grow to your first 1000 email sign ups. No really? Yes, you could launch here on Product Hunt, however, that is not a sustainable marketing channel, its a one off hit and run. Besides this most product hunt launches I see, are beyond this point already, they have a small userbase with the product somewhat validated and built. How do you actually do it, has anyone got real examples? Especially those people who get 1567 signs in their first week.... Please do share.