Earlier quoted context omitted.
They wanted to build an open, paid version of the Twitter API/ecosystem. But the value of the Twitter API lay in the access to their 100 million+ users, and all the valuable social data generated by those 100 million+ users. No amount of "open infrastructure", no matter how clever can ever replace that. It's like buying 1000 data centers in every country, and building an entire system that can handle a billion concur…
It's not open. It's closed, proprietary and the boys over at Andressen-Horowitz are going to want a return on their investment.
And they have started releasing client source code: https://github.com/appdotnet/
a16z is already getting a return, since App.net has been profitable for a while. It's all gravy since they've made it free to sign up. The user base is about 10 times bigger than last year. It's true we don't know how many new users are paying but since that fixed costs are covered, it doesn't really matter.