As I coach and consult with clients, when we discuss job search techniques, I emphasize the importance of being careful with one's online presence. Not only do potential employers google the applicants to gain information, but now also scour the web, social networking sites in particular looking for digital dirt, and using this as a basis to say "no thanks" to an applicant. In fact, the Federal Trace Commission has authorized Social Intelligence Corp of Santa Barbara, CA to sell reports on checked applicants to employers and those files can last for seven years.
Be also aware that you might be very careful, but your Facebook "friends" might not be so. You could indeed be linked to someone nefarious and be cast aside due to guilt by association. Remember, this is legal; right now this company uses human analysis; however, before too long, this too may be "outsourced" to more and more hi tech software.
What can you do as a jobseeker to "remedy" this? Right now, not much. Your best bet is to clean up digitally as much as you can, and then blog, write articles etc. Establish yourself as an expert in your field, a "go to" person who writes authoritatively and well. At least as you add positive items to your web presence, you can push down any others that can potentially hurt.