I feel like I shouldn't even have to post this. Maybe it's not a lack of common sense but just naive misunderstanding of how "laid back" and "cool" the ad industry is. When you're looking for a job, or applying for a job, or interviewing for a job, or heck, anytime--don't be an idiot online. Almost every company will do a background check on you, by which I mean Google your name. And while ad agencies might be more forgiving of those wild party pics on your Facebook profile, why chance it?
If you're going to blow a job opportunity, do it the old fashioned way--in the interview. Set your profiles to private while you're looking. Or better yet, don't put anything on any site ever that makes you seem like an idiot. If a friend of yours tags a photo of your bare ass hanging out in their "Fun Times At Albany Bowl Sat Night" album, kindly ask them to pull it down. This goes for interns too. When you work at an agency and become BFF on FB with all the folks there, then return to school, a year's worth of drunken photos and mobile status updates about how wasted you are at In 'n Out can start to undo a summer of responsible hard work. Just the way it is.
Here's a fine example. Job offer --> Stupid Tweet --> Poof! No job offer anymore.