Nathan,
This is precisely one of the reasons why building relationships prior to graduation is so important. Social networks are collaboratively developed databases of people that are user generated. The information is updated by the user and if Career Services has purposefully built relationships with graduates, it provides yet another tool to build rapport with students, stay connected, mine for data which provides insight on their employment, activities, and personal characteristics, from which to develop effective communication strategies. It's completely unlike any other strategy typically used by Career Services - it's long-term and it's continuous. Perhaps one of the reasons it is so rarely used which is unfortunate because it is so highly effective.
The graduates I worked with were primarily freelance and non-traditional employment as well and our social media strategy helped us improve employment rates as a result of the expanded and improved relationships we had with students who were more willing to disclose their employment information. Additionally, we had more opportunities to discover when students/graduates were working because of data mined on social networks. It allows you to optimize your ability to address the challenges you describe.