A company should view information about applicants on Facebook and other such sites because this information would tell them valuable things about how the applicants present themselves to the public and about how well they are likely to do as workers.
People will generally be on their best behavior at job interviews and will put their best foot forward. This makes it difficult to truly know who will be a good worker and/or a good representative for a firm. But Facebook and such sites offer greater insight. They show more of the applicant's true personality and therefore tell the firm more about what kind of worker they might be hiring.