“Yahoo ranks as the most-visited website in the U.S., according to the latest report from ComScore” (So: ‘US’, not ‘the world’)
and: “Traffic to Tumblr [...] did not contribute to Yahoo's overall traffic number[...] Tumblr ranked as the 28th most popular U.S. property” (Again, just the US, and not even that popular)
It's Yahoo properties, not Yahoo.com. Big boost: they bought Tumblr.