This is self-reported. Consider that 90% of Croatians reported they speak English, but this is mostly the level required to guide a tourist to the closest bar around ;)

EDIT: my bad, you were talking about native speakers.

EDIT2: well, no, my point still stands: the Total includes self-reported and vastly inaccurate additional language speakers. But based on how US is 2/3 of all native speakers, and that there's definitely many fluent-English secondary-language speakers out there, US is definitely not dominating English-language web.