A lot of people don't want to be the product and believe when a company focuses on them as a customer rather than them as eyeballs to be sold to advertisers, who are the customer, then a better service is the outcome for the users of the service.
With app.net the user is the customer. With Facebook and Twitter, the user is the product. With App.Net user interests and service provider interests are aligned. The provider wants the service to be better for the users.
With Facebook and Twitter and other ad supported products the users who value their privacy have intentions which are constantly at odds with the service provider whose intention is to continually open up details about the individuals so that those details can be used to improve ad success rates and profitability.