Note: This is a condensed version of a report that will be included in the upcoming edition of the Wikimedia Quarto.

Wikipedia is now the most popular reference website on the internet in terms of market share, according to data disclosed last week. Confirmation of this milestone came thanks to a report donated to the Wikimedia Foundation by Hitwise, a company that tracks internet and search engine usage.

According to data provided by Hitwise, the Wikipedia.org domain surpassed Dictionary.com as the most popular reference site for US internet surfers in late May. In the two months following this achievement, Wikipedia has strengthened its lead by nearly two percentage points. Wikipedia's market share in the "Education - Reference" category now stands at 5.86%, while Dictionary.com has dropped to 4.01%.

Also of significance is that Answers.com, which includes content from Wikipedia as well as other sites, has gone from not registering at the beginning of 2005 to a market share of 2.33%. Among other encyclopedias, Encarta comes in at 1.32%, while Britannica does not appear in the results, presumably because its content requires payment to access.

Alexa traffic rankings also track individual categories, including one for "Reference". There Wikipedia ranks second, but the logic of the category is difficult to understand, since the first spot is held by My Yahoo!, which is really a customizable portal page.

The Hitwise report indicates that of all traffic coming in to Wikipedia from outside the site, 66% originates with search engines. Breaking this down by source shows that out of the search engine traffic, 50% comes from Google and 43% from Yahoo. MSN Search only brings in 3%, an unexpectedly low figure given recent reports on the market share of the search engines themselves. Yahoo performs better than its market share would suggest, which is not surprising given that Wikipedia was included in Yahoo's content partnership program over a year ago. MSN, which is closely tied to Encarta, obviously has no such relationship, although Wikipedia articles do seem to show up reasonably normally in its search results.

Wikipedia's visibility on Google has increased significantly in recent months, as the Google web crawler went from having 3 million Wikipedia pages indexed in March to more than 25 million in July. Pages from the Wikipedia.org domain currently occupy 0.32% of all pages in Google’s index, a number that matches up closely with the proportion of search engine traffic that Wikipedia attracts, which is two-tenths of one percent. Wikipedia is now the 22nd most recommended site among search engines.

Among the thousands of keywords for which Wikipedia ranks high in search engine results, several in particular consistently draw considerable traffic. These include Bobby Fischer, Jacques Chirac, Vladimir Putin and, most notably, Pope John Paul II. With Wikipedia having been established as a popular source of information about the papacy, the election of Pope Benedict XVI caused a large influx of traffic, peaking at 2,100 requests per second. The number was spectacular at the time, but as hardware capacity has expanded, only two months later it is common for the servers to receive over 2,500 requests per second.