First off, the landing page with a video explaining the problem and proposed solution is a great lean approach to getting initial validation prior to building the product. If you can, mockup some lo-fi wireframes of your idea for the webapp in Invision or Axure. Then get some prospective users to test your idea. As was already mentioned, the hard part is finding those initial users that are interested in your product. Lastly, you say your solution is "too complex to prototype on my own." That's a red flag that you may be trying to do too much too soon. I would take a step back and really spend some time thinking about the simplest solution to the single problem you are trying to solve. If you can identify a single piece of your webapp that you can get out in the market and gain traction with, do that. After that, you can start to build out the other features and have a customer base to solicit feedback from. I've seen buttons for future planned features in some apps that just pop up a window asking the user to click "Yes" if they want that feature in the product. That's a great input when trying to prioritize which features to build next. Good luck!

My favorite phrase as it relates to this is: Fake it, til you make it! The question I would ask is, can you be hyper-focused on a specific small group of users, and seed content specifically for them? Facebook started with one specific school, and seeded itself with information on classmates. When people joined, they found it engaging, and kept coming back. Identify a group in your city and approach them about using your platform to solve a problem they are having. Get them sold on your platform. Get them onboarded. Work with the leader of that group to ensure there is content and activities for those users. Use that group as your alpha group, and learn from them what works and what doesn't. Validate your platform is a solution they are happy to use. Then repeat the process with a new group. It will be a very manual process to start, but that's how you'll get those initial users on your platform. Just keep grinding away, and it will start to grow.

When we started TakeLessons, the first thing we had to do was understand if our product resonated with users. There were a lot of different strategies and partnerships that lead to our ultimate success, but first and foremost is ensuring your product meets a need in the marketplace. Focus on customer satisfaction. In the early days, you treat each customer with a white-glove approach. Speak to them. Help them. Learn from them. What did they like? What didn't they like? What could you do better? Then, take their feedback and iterate your business/product around it. Once you've made some changes, go back to those customers and ask how you did. Get their feedback. Then repeat the process, but include new users, with fresh perspectives. What this does is two things. First, it gives you direction on how to build a better product. Second, and maybe more importantly, it shows your customers that you are there for them, and willing to put in effort to do things for them. These customers will turn in to advocates of your business, spreading the word about it, (ie increasing sales), and help to increase your customer lifetime value as they keep using it. Keep doing this, and over time, you'll have a great business, with customer-focused products. Happy to discuss more in depth.