How did people's attention spans end up becoming so short?

From Facebook, to Twitter, to this very website, it seems that people have huge difficulty paying attention to things. They just jump from post to post, and, when they comment, their comments are so short and often don't have any punctuation. They just type some badly-worded nonsense and then jump to the next post, only to do the same on that post, and the post after that, and so on. How did people get this inattentive when frequenting the Internet?