[ggreer]

When I visit Facebook I see...

    font-family: Helvetica, Arial, sans-serif

...on every element I inspect.

When I visit Wordpress.com and check the font on the body, I see:

    font-family: -apple-system,BlinkMacSystemFont,"Segoe UI","Roboto","Oxygen-Sans","Ubuntu","Cantarell","Helvetica Neue",sans-serif

For paragraph text such as on https://wordpress.com/about/ I see...

    font-family: Merriweather, Georgia, "Times New Roman", serif

...though no web fonts are downloaded.

It's wordpress.org (not .com) that has Open Sans. Not sure what that's about.