So have you ever Googled your own name? Do you think anyone else has ever Googled you? If you are living in a fantasy world then you are truly underestimating the power of Google and answered "No" to both of these questions. Everything that is posted on the internet can be found through Google if you don't use the proper pre-cautions (privacy functions available on most social media platforms). Trust me, if it's out there on the internet, employers can & will find it.
If employers are able to freely search for you on the web, they can find out about your personal relationships such as marital status (or in some cases if you are homosexual or transgender), they can look up where you live and view your house on Google Maps, they could find out if you are pregnant via status updates on facebook or twitter and they can absolutely make judgements about the types of people you associate with.
Of course, all of this falls under categories which employers can not discriminate against but that doesn't mean it doesn't happen. So the next time you make a status update, post a picture or Tweet, think about how you would feel if a potential employer were to see it.