>> Twitter's ad targeting depends on knowing what a user looks at

But they can track which tweets are being clicked, the IP of the person and a whole bunch of things even when you're not logged in.

>> and how their network of connections/follows

But that's the problem right there. If you force people to create an account to read tweets, they might do so but they won't follow anybody or follow very few people and tweet quite infrequently. I just don't think it's feasible to expect a significant number of people (like say # of users on Facebook) to be tweeting regularly. But it's possible that I'm wrong.