This is too bad. I remember learning about 21.co (later earn.com) and being excited about the idea of using a pricing mechanism to eliminate spam and increase the quality of my inbox. This core idea is a big part of what I'm working on right now [1].
One thing that 21.co never really figured out was a killer use case, or how to best connect buyers with highly-targeted sellers in this "attention" marketplace. It just seemed like a cool way for startup CEOs to get better quality emails and direct the money to the charity of their choice.