Finding the Right People
So how do you know what alumni from your school work in media? One of the best ways to find people is through your college career center. Often the career office will have binders -- or, now that we are in the 21st century, an online database -- with information on grads working in a variety of fields.
Flip through the media book and take down info for people working in jobs that strike you as interesting. Once you have that info, start sending out emails. But, before you dash off any notes, keep in mind some of the rules about how to network, specifically on crafting those introductory emails.
Why Alumni Make Good Resources
The premise behind alumni networks is that they exist expressly to help people in your situation, i.e. grads looking for jobs. While not every alumnus will be willing to help, most will. Especially those who have offered as much by giving their information to the school as a resource.
That said, keep in mind that the people you're contacting are busy professionals. Look to them for information and thank them for everything and anything they do for you, whether it's just responding to your email or helping you land an interview.
Further Reading
Middlebury College has some good information on its website about how to network with alumni.