Interviewing is always a delicate topic. There are things you cannot legally ask. However, in the age of Facebook and Twitter, people freely post personal information disclosing information that an employer could never ask about. In fact former Google CEO Eric Schmidt advised young people to change their name to escape the on-line shame of their youth.
I have been involved in a number of situations where an organization's employee has posted inappropriate pictures or information on their personal social media accounts. This has often created a personnel issue inside the organization. Having a daughter on Facebook I now get to read many public posts by a number of teenagers and young adults that some day someone at the college they want to attend or a prospective employer will read. Many are not flattering. That is a very hard concept to instill in a teenager, but that is the reality of the internet. Law enforcement organizations routinely scour social media to find out who is associating with people they have under investigation. As an employer social media posts may fill in the blanks as to what type of person is sitting across from you in an interview.