The data are from Alexa and the countries with “unknown” best I can tell do no produce enough data to be ranked. Google’s country domains (.ca, .jp, etc.) number 200+ and are all included as “Google.” No properties are included (flickr.yahoo.com for example).
Some things do pop out:
Russia and China like their websites homegrown. Russia’s Yandex is a search engine as is China’s Baidu.
Cameroon and Japan prefer Yahoo. Do those countries have anything else in common?
Why does Kazakhstan like to e-mail so much?
What other questions does this map raise?