He postulates that in the past few years we have all been discussing how social networking and social media had transformed the way we consume online communications, and, therefore, it was safe to assume would transform use of email
So Neilson, having the time, resources and data available decided to test the hypothesis that “Consumption of social media decreases email use.”
They defined the online world as four groups by social media usage, three by use in minutes, the fourth non users. And then looked at each groups email consumption over a year, and subtracted the email time of the non-use group to give a flatline average use.
Here’s what they found:

So from this admittedly rather rough and ready calculation the more people use social media the more they use email.
So why? To some degree this must be people getting updates from Social Media sites like Facebook – the ,more you use social media sites the most updates and flag emails you are likely to get
Of course it is also true that the really heavy users of Social Media are likely to be the “power users” across all online media, spending more time online, on email on Facebook and all the rest. As we are all seeing on Twitter we are actually all talking to each other.
But that doesn’t quite explain the growth, or the monthly variations. Neilson have promised to do a more in depth analysis which takes into account offline email, direct messaging, alerts and all the rest, and in a more scientific and rigorous test.
The results will be interesting and definitely something to watch for those of us trying to find the optimum way to communicate with supporters and donors via the web.
Is Social Media Impacting How Much We Email? Jon Gibs, VP, Media Analytics, Neilson
http://blog.nielsen.com/nielsenwire/online_mobile/is-social-media-impacting-how-much-we-email/