Email is not dying in the midst of the social-media revolution. As one social-networking company said to me, "accurate delivery of email is a main part of our deliverable."
Flipping that situation around: How can we, as emailers, best leverage the new social-marketing applications?
Let's consider the simple email newsletter and expand it to include social marketing.
The first thing you should do is to consider the wordiness of your writing. Make it short and snappy, as your readers are likely on a mobile phone and unwilling to scroll and zoom, or tilt sideways, on their iPhone.
Include plenty of links to follow up and read in more detail. Keep images meaningful, well-labeled in alt-text.
Don't clutter it up with images solely used to manage layout issues (like spacer GIFs or transparent pixels), as they will appear broken and messy. The single-pixel GIF is OK if kept at the footer.
Recognize that they're subscribing to more than email