Ask HN: Where do you find good testers?

We've been working on a new webapp targeted at news junkies and smart(er) people. We're at the phase where we're using it internally and it seems (relatively) stable. Not quite ready for an HN launch, but we need 30-40 quality testers so we can look at some performance data and get some feedback.

How do you round up testers for your apps? We've got CS undergrad profs handing out invite codes at the local college, and we've enlisted several IRC hangouts we frequent, but so far results have been underwhelming. We've got a few really promising testers, but social apps tend to require a 'critical mass' to get things rolling. Where are all the testers hiding?

(If you want to sign up, http://tinyurl.com/newsbeta). More interested in stories about finding testers for your webapp.