Well, most of the "secret sauce" is wrapped into the algorithms that we've built so I can't say too much. In general, we're able to learn what topics you're interested in via data that we collect through your implicit and explicit actions. (i.e., we're learning about you by watching who you tend to interact with, retweet, what topics you tend to click through on, who you follow, etc.)

We've been building and improving the algorithm for a few months now -- it's certainly got a long way to go still but, on the other hand, it's certainly come along way so far. :)