From Scientific American:
Consider two questions. First: Who are you? What makes you different from your peers, in terms of the things you buy, the clothes you wear, and the car you drive (or refuse to)? What makes you unique in terms of your basic psychological make-up—the part of you that makes you do the things you do, say the things you say, and feel the things you feel? And the second question: How do you use the internet?
Although these questions may seem unrelated, they’re not. Clearly the content of your internet usage can suggest certain psychological characteristics. Spending a lot of late nights playing high stakes internet poker? Chances are you are a risk taker. Like to post videos of yourself doing karaoke on YouTube? Clearly an extravert. But what about the mechanics of your internet usage—how often you email others, chat online, stream media, or multi-task (switch from one application or website to another)? Can these behaviors—regardless of their content—also predict psychological characteristics? Recent research conducted by a team of computer scientists, engineers, and psychologists suggests that it might. Indeed, their data show that such analysis could predict a particularly important aspect of the self: the tendency to experience depression.
More here.