You should be aware that once information about you is on the internet, it is out of your control and likely to be able to be found for a very long time. You should protect your privacy and reputation by taking these steps:

Set appropriate privacy and security defaults and choose a complex/unique password.
Be careful installing third-party applications.
Only accept friend requests from people you know directly.
Read the privacy policy and terms of service carefully. Limit personal information you share.
Consider all information and pictures you post as public!

Even more tips at: http://socialmediasecurity.com/

Duke Careers' tips on protecting your online reputation: http://www.slideshare.net/DukeCareers/online-reputation-10486586