SA and 4chan both need to be much higher. They created 90% of the internet culture there is today; Hell, they created the modern concept of internet culture. Good selection of websites over all, bad order. Echoing others, but wikipedia being number 1 is definitely on point though.
I mostly agree, but sometimes I think people overestimate how influential they are, there are so many websites that influenced the web that barely have a trace anymore. For example, they mentioned GameFAQ's LUE, which during its heyday was like an early version of 4chan that spawned a lot of internet culture. However well over 99% of it doesn't exist anymore and is probably lost forever. It was around well before people really archived websites and took screenshots so only a few examples from it survive.
I'm sure there are tons of other influential websites that exist that are practically lost to history.
On the other hand SA has an archive and 4chan has some of it archived and people take many screenshots of what happens there.
Also, The Onion is up waaaaaayyy to high on the list. Additionally, its funny they only mention one token porn site, if they were being honest there probably would be way more on the list.