What is email - skewed definitions skew results?
There is a world of difference between "email" and "web based email", with the greater number of people that I know using "real" email - a client and a mail server - rather than a web version.
Design of a study is paramount in getting meaningful results. This looks like a case of taking a metric because it is "easy to measure" rather than trying to capture the full spectrum of online activity.
Useful definitions are crucial. The amount of time spent writing and reading email is hard to measure because the client typically connect to the server only to check for or transmit mail. Thus,.in my case, there is a connection for a few milliseconds every ten minutes, interspersed with email uploads and downloads. I, however, am continuously engaged in reading and writing.
If you are trying to measure user involvement in an online process, I am continuously engaged, though it is not as easy to measure as time spent on a web site.
A more useful comparative measure might be the number of words posted on blogs and on web sites, compared to the number of words emailed. That, however, would emphasize sending over receiving. Counting messages is only relevant if the approximate message size is similar.
And someone clicking on a web page does not actually mean that they read anything... being "logged in" to a web site is equally meaningless. I am often "logged in" to a social site, but the window is buried on my desk top for several hours. Does that mean I get credited with several hours of social networking, rather than the 10 minutes it actually took?
These "high level" reports without methodology, design, and anything but statistical reductions of data are too often meaningless or misleading. Without the survey design, sampling scheme, questions, statistical criteria and analytical methods, and survey methodology there is really no way to judge whether anything in the report is valid.