Revenue model based on ads are quite difficult when the prime reason for the user is to consume and post content. That is the issue of Twitter, and was the issue for Facebook before they invented all the new ways of earning and introduced all these ad possibilities in their mobile app. The 140 characters is kinda limited, but keep in mind large part of the world is not connected to the internet, but uses voice and sms. I have no idea how many of them use Twitter, but sms frontend to internet services is still something that is very interesting. using USSD the UI for sms(like) frontend is even more simplified with text based menu structures; But USSD is not really big, but still pushed by many African telco operators for instance.