Ask HN: What's the Best way to get private beta testers for early feedback?

I have been working on and off for the last year coding a new social based platform that is focused around the usage of hashtags. My biz partner and I have just recently decided to work on this fulltime and have been working day/mostly night working out some of the bugs and are now starting to focus on cleaning up the UI. We are almost ready to launch a private beta to finish flushing out the UI based on user feedback.

What is the best way to find beta testers that will give us the feedback that we need to take this project to the next level? Are there any sites where we could showcase our start-up for beta testers? Do you have any other suggestions? We are looking for around 500-1,000 beta testers that will really take the time to give us honest feedback.

We threw together a quick video that explains the platform for a local start-up accelerator which might explain more about our service. http://www.linkies.com/video.html