Here is an interesting information published ny emarketers :
According to ExactTarget’s “Subscribers, Fans and Followers” report, there is little cannibalization among the three channels—instead, there is significant overlap. Most internet users studied in April 2010 engaged with brands only via marketing emails, but nearly a third subscribed to emails in addition to being fans of brands on Facebook.
This means that socialmedia clearly is a way to further engage your readers, transforming them into fans and followers and further deepen the relationship you have with them.
Emails does not replace social media, it is complementary.