Emails require more of my attention and they arrive whenever they are sent. Tweets require zero attention, they are easy to ignore, and I never see them unless I specifically seek them out by opening Twitter. Basically Twitter is much more voluntary than email.
I get this, but it's kind of funny to hear someone say Twitter's unique value proposition is that you can ignore it. As a non-user I must be extracting tons of value from it then! It's the poor users I feel sorry for!
Just joking around. But, not completely. The value you speak of is really the ability to spend your time on something potentially more worthwhile. People forget social media has an opportunity cost.
