-- Prototype --
I would add that you can create prototypes more simply by just using static images and adding interactive elements to them. I recently discovered MarvelApp (https://marvelapp.com) and you can create mobile + web prototypes for free with them. I would recommend hiring a designer to make the prototype look impressive if you have the funds.

-- Messaging --
On your site you want to be clear about the problem you are solving or the benefit you are providing to make people's lives better. Answer the question - Who can't live without your product?

If you have a target audience in mind, provide examples of how your product would integrate into their daily life. Provide examples and try to be as concrete as possible.

If there are other big names doing something similar to you - mention how you are different and superior so they can frame your product in their minds.

-- Traffic --
I think Andrés' ideas on getting traffic to your site are great. I would add Google Adwords or Bing Ads to the list if you have some cash to play with. In addition to bringing more traffic, this is a great way to test out which wording resonates with users the most so that you can use it on your site. Online Ads also let you play with targeting different demographics to see where you get the most interest from.

Visual prototypes can go a long way for validation. I recently validated a product Idea by:

- Sharing the prototypes with everyone I could get to meet IRL, asking for feedback and what they thought about the idea.

- Created a landing page to gather signups and show a bit about the product.

- Posted visual designs on dribbble, HN, reddit, twitter, etc (to your relevant target audience) - this feedback was invaluable as it comes from people who are not attached to you in any way. You may also get lots of signups this way.

- Promote the landing page in relevant communities, be straight forward and say you are validating your idea - even more signups.

I was still building my product in the background, but reaching over 900+ signups in a few weeks for a product that was not even out there counted as idea validation with just a set of visual prototypes.

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!