Urban Doctionary gets around 30x the traffic. So it definitely gets the lion's share of slang dictionary traffic.

The Online Slang Dictionary was always a side project from when I started it in 1996 until 2007, when I started working on it full-time. It wasn't until 2007 that people could put content on the site without my manual intervention. So it's easy to see why Urban Dictionary overtook my site in the long past. Since 2011 though, Urban Dictionary's traffic has been artificially propped up due to an ongoing manual penalty against my site, which I've had no success in resolving.

So if anyone works at Google or knows anyone who works at Google - I can be reached at waltergr@gmail.com or editor@<my site's domain>. :)