This is THE BIGGEST MONEY MAKING IDEA IN TECHNOLOGY full stop. The money that will be made around this will make Adwords look like a cute hobby. Messaging on all channels needs to be assigned a variable cost. ...and this implementation is thoughtful/interesting, as is 21.co... Google, Facebook and Microsoft have understood this is coming for a long time. That's why Microsoft bought Linkedin, and FB bought Whatsapp. Companies are spending billions to prepare for this future, but I have yet to hear anyone elucidate how this paid communication future will actually work. Further, since the patent system has been eviscerated there isn't any incentive for an entrepreneur to solve the whole problem... and Google and many other tech companies only know how to copy good ideas... not generate their own. So your reward for starting a company that solves this will likely be lawsuits, poverty and personal and professional devastation. Who is gonna step up ;)