The Fonts within Google's free Web Font Library have been seen over 7.6 trillion times.
Google provides some analytics on the fonts within its library which highlight not only the total number of font views, but the number of views per font in the past 7 days, 30 days, 90 days and one year, and eve offering up some data points on how specific font use changes over time
According to the site, Open Sans is the most popular font by a huge margin, being viewed more than 1.8 trillion times, followed by Roboto at 1 trillion andOswald at 367 billion.
Google also provided a font view distribution by browser and operating system. That's useful as it shows designers which fonts are most common on specific devices and system setups.