16,641,486 websites loads Google Analytics code library which means they are using Google Analytics according to this search result: https://search.nerdydata.com/search/#!/searchTerm=google-analytics.com/ga.js/searchPage=1/sort=pop
And 265,995 websites has s.pageName on the web pages which indicates they are using Omniture SiteCatalyst according to this search result: https://search.nerdydata.com/search/#!/searchTerm=s.pageName/searchPage=1/sort=pop
Considering Omniture is so expensive, it is actually quite a big number. Note that one client could have multiple websites so it is not saying that Omniture has 265,995 clients. Anyway, very interesting figures.