This an interesting question. Skimming the list it seems that - at least - it is not based solely on popularity (like the Alexa 1 million list for example).

I see lots of websites for small German towns, which should only have a couple of visitors per week.

Of the blogs and personal websites I read - some popular, some less so - only paulgraham.com is there.

I suspect the list is biased to official and very old domains.