The traditional dumping grounds were Pierce County (mine :( and Spokane.
I had not heard that particular figure...but I know there are a sizable number (it's covered quite a bit by local newspapers, one of which I remember one time said that there were "many thousands" of neighborhoods that had registered sex offenders living in them. It was an article about the massive job of registration...and boo hooo, the poor under-funded state employees who "had" to do it -- like they should get paid for filing their nails or something).
What I don't know, is what level of sex offender...that tag can range from just nasty characters to truly dangerous beasts.