This really is a discussion of one of my favorite topics - correlation vs. causation. Just because two things correlate ("Many Irish people have red hair" ) doesn't mean that there is a cause and effect relationship ("Having red hair causes Irishness). While she found correlation, she did not mean to imply causation, but the media jumped on the story.
So, it's ok - college students won't lower their GPA through Facebook use. So if you're paying a student's tuition, don't worry about Facebook being the problem. It's just another tool that they can use approriately or not, just like radio, TV, the Internet, and alcohol. Good luck.
Infographic by College Scholarships.org