What have you done?Basically - pretty much the exact opposite of what you've done :-)
I had an idea some time back. I went and found some users first. Then I observed what they did and what they valued. Then I changed my idea. Then I went and observed some more. Then I changed my idea again. Then I went and talked about the idea to a group of folk that I was 90% certain that I was solving a problem for - and they mostly liked it. Tweaked the idea a bit. Went back. They loved it. They told their friends. And they pre-registered. Based on word of mouth.
Now I've got to the point where I can build something.
I know I have probably a couple of hundred users who are keen on the product. I know that they're keen enough about the idea to recommend it to friends. I'm not entirely sure whether they'll be entirely profitable or not - not sure yet. The only way to find that out is to build something (or the quickest and cheapest way anyway).
The point I'm trying to drive home is that having a way of getting to users is a core part of your business. It's not something that you should wait to figure out until after you've built your product (because figuring it out may well affect the way you build you product).
What would you recommend in my shoes?
Find a group of users of your product that you can contact directly.
In real life - face to face.
Preferably find a sub-group of your market that is most passionate about whatever problem it is that you're solving. The ones with the most pain, or the ones with the most to gain.
Talk to them one-on-one, encourage them to use your product. Watch what they do. Where possible tweak the product to support that group in a total wonderful way.
Encourage testimonials and references (any forms of social proof that you can use before you get large numbers).
Encourage the people using your product to invite other people who will use the product. Look at the way FreeAgent and Rackspace referrals work for example.
Be prepared to put a lot of effort in.
In my experience getting actual real users is a long, hard slog that takes time and effort to pull off. Unfortunately I think you're actually at a disadvantage now by having a fully formed product without users - because you may find that the "generic" product that you have now is harder to pitch to the smaller market that might have pulled in your initial early adopters.