Twitter have just announced the ability to add an email signup form to a promoted tweet, using the new Lead Generation Twitter Card. Aside from being able to add text to the tweet, you can also add an image, some call to action text and the signup form itself.
The form is pre-populated with the email address the user signed up to Twitter with. In addition to the user’s name and email address, their twitter username is also submitted – so if your ESP is capable it means you could start to do some clever analytics based on the user’s twitter feed.
Google have dabbled with this, allowing email signup via adwords in the search results. Some of our clients use that and it works well for them, the challenge then becomes to get more data out of the subscriber, such as name, gender, location etc.