I think people read into this ''online presence'' thing far too much. For example, Oxford alone received 17,480 applications this year. In 2011 a total of 492,030 students accepted an offer to study. Imagine how much time it would take to look up all of these people, time which could be spent on something that will actually benefit the university.
Think of all the applicants who are named John Smith or something equally generic, it would take forever to find the right person. Anyone with common sense will have all their settings set to private anyway. If someone who i'm not friends with managers to track my Facebook down, all they'll see is my cover photo. I've noticed as well that a lot of people are using their middle name instead of their last name, presumably to avoid detection.
Besides, what on earth are they even looking for? What could a 17/18 year old post on there that is so bad that it would cause them to decline a perfectly able candidate?