Kristine,
It is true that LinkedIn brands itself as a "professional" network while Facebook has a brand more associated with "social" and less "professional." The truth is any network can be used for professional purposes. It is not the platform that determines how one should use it so keep this in mind when using any platform - you are in control and can use it with purpose based on your goals. Facebook can certainly be leveraged for professional use. For example, I often tell job seekers to not only look for jobs but to look for people and the right people can lead to the jobs. Thus, networking as a job search strategy (and a perpetual career development strategy) really is dependent on building a quality, targeted network. So, social media expands one's capabilities to strategically architect a quality, targeted network no matter the platform (Twitter, Facebook, LinkedIn, etc.) Sometimes, it isn't the big networks that make the most sense - it depends on your audience.
I'd like to play devil's advocate here. Rather than "using Facebook for graduates because of it's more 'professional' aspects," as the stated reason for choosing this platform, what is the greater objective? What is LinkedIn being used to accomplish with the alumni audience? The same applies to the use of Facebook. What are you trying to accomplish among your student audience that drives the intent of using the Facebook platform? Can you reflect on this and provide your thoughts regarding the goals you'd like these social media tools to help you achieve?
Robert Starks Jr.