^Exactly. There's no time for that stuff, no way to get distracted. Things like those don't matter anymore, you're not thinking about wasting time on the internet. You want to get your shit done and leave.
As a student even if you have an exam, it feel like you have time and studying for that is not mission critical in that very moment. Being on Facebook for another hour instead of studying just means you'll get 5-10% less awesome for your exam, or delay getting awesome. At work it means accomplishing something or failing hard, and facing consequences that go beyond a lower grade or having to retake an exam.
That being said.. sometimes in between important things I lurk ;> Once I was even late for work because of SH - a reply took longer than planned, lol.