I'd ask questions such as:
-Describe your experience using social media in a professional setting. More of an essay.
-You can ask questions about their social media activity? Do you have a personal blog? Have you ever run an organization's social media presence? Blogs? Twitter? FB? etc?
-Could also ask about specific tools? What would you use to maintain multiple social media presences? Track for records management?
-How do you measure social media ROI?