Is it as simple as: Take the first occurrence of "/" and then count backwards till the 2nd ".".

Therefore: www.wordpress.com/12345-65758 = wordpress.com john.wordpress.com/12345-65758 = wordpress.com 123.john.wordpress.com/12345-65758 = wordpress.com

============ If that is the case, quite a few blogs (wordpress, tumblr, posterous etc...) would only show by the blog hosting service domains (e.g. tumblr.com, posterous.com, wordpress.com etc...)

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But this doesn't always seem to be the case: http://news.ycombinator.com/item?id=592268

Is there white list of domains to count to the 3rd "." ?