Interviewers will look you up online. I've had interviewers reference my Facebook, Twitter, Wordpress, and LinkedIn. You have to be prepared .. Think of these as opportunities to tell more about yourself that you may not be able to express in an application.
Also, people can fake the tone of an essay, but it's much much harder to produce years of 'doctored' online content .. So for an interviewer, looking you up is a great way to see what you're really like. Use that to your advantage!