There's a long list, including private correspondence, commercial journals, proprietary databases, trade secrets, internal corporate data sets, private archives, financial trade data, classified national databases. That was before the rise of FAANG, big data, proprietary analysis / inferences / knowledge graphs derived from public data sources, metadata traffic analysis, and advertising surveillance business models.

I can't find a reference at the moment, but this topic was covered in a professional journal for historians.

Not sure if this is what they meant but email is thought to be larger in aggregate than the web.