It’s probably hard to separate out the exact effects of each of the different channels from the outside, since when a campaign is communicating via soc net site, whether to promote a video, a fundraising drive or just the message of the day, the staff is also almost certainly also sending out related emails to their list (perhaps several times more emails, too, and with targeted messages).

It’d be fascinating to get internal numbers to measure how the results compare among promotional tools like email, sms and soc nets. In any case, a campaign like Obama’s has the resources to reach people by whatever channel the supporter chooses.