There are two major aspects to such an argument.
First, you can argue that an employer has the right and the duty to select the best possible employee. The employer should therefore seek out any information that is available to them through legal means to ensure that they are not about to hire someone who has serious character flaws. Screening potential employees is simply something that firms must do to protect themselves against the waste of resources that occurs when a bad employee is hired.
Second, you can argue that any employee that you hire will become part of the public face of your firm. If you have an employee who is publicly bragging about bad conduct, your firm will look bad. People will have a hard time feeling good about your firm if you are willing to hire someone who puts themselves forward in extremely negative ways via social media.
A business can be harmed by bad conduct on the part of their employees. Therefore, businesses (you can argue) should delve into applicants' Facebook and other social media to determine what kind of person they are potentially hiring.