Ah, a great question... It really depends on you and your field of work, as I think creative fields often allow their employees to have a little bit more fun than workplaces where wearing patterned tights is considered edgy. And if you are applying for a job specifically in social media, you really need to have a strong, professional online prescence. (I get suspicious when I can't find someone anywhere online.)
But, with all of that aside, you really want your employer to judge you on your resume, references, experience and interviews -- not your charming YouTube videos. If you are already have too much of a life online, just go through and edit down anything that's too edgy or embarrassing (and maybe get rid of that high school MySpace page). Use your privacy settings so that random people can see your profile picture and maybe a little info about you, but not your family holiday pictures.
You want your online persona to show that you are an interesting human with a life outside of the office -- but you never want that to overshadow your qualities for the job.
Gabriel Shaoolian of Blue Fountain Media wrote a column about polishing your online persona. It was aimed at students applying to college, but the advice applies to job applicants, too!