We recently explored the (simplistic) question of whether social media is killing email, and showed how the two channels are not mutually exclusive, but complementary.
For those looking for resources addressing the issue of how email marketing is changing in light of evolving Internet habits and preferences, we recommend this treasure trove of links to online articles on the subject titled tongue-in-cheek "Email is Not Dead."
One interesting quote: "Email is not dead because people want to write to one another, using their email accounts, in more than 140-character bursts." This is so true - I frequently find myself struggling to squeeze an important message into the 140-character limit of a text (and using all sorts of abbreviations like "c u @ wrk" and "r u going 2"), and then realizing, "Hey, I know (or have) this person's email address," and then quickly switching to Blackberry email mode to compose a more intelligent missive.
Obviously, there other insights are provided as well. See the articles at the link above.