Created attachment 38642 [details]
I haven't been able to figure out how to reproduce this beyond "browse normally with a WebKit nightly build for a while". The attached crashlog is representative, but it doesn't always crash. Sometimes it just spins up a ton of threads for a while, then quiets down again.
Created attachment 38700 [details]
I could never reproduce the crash, but this should greatly reduce strain on CFNetwork.
Magic numbers used here are not special or tested in any way. I did try browsing with some additional logging, and both CFHost calls and network requests made seemed reasonable.
Fixed in <http://trac.webkit.org/changeset/47845>.