Good question Jocelyn - Our referral campaign is tailored for full automation and this reverse lookup operation is not supported today. Here's how the referral offer (and other social apps) work - When Jane signs up, she gets a short URL (say, http://mrk.to/jdoe) which she forwards to her friends. When Joe clicks on the link AND fills out a form, we populate the Social Syndication Id field in Joe's lead record with a machine-recognizable identifier which ties Joe back to Jane, his referrer. This machine-recognizable id is mapped to the short URL (http://mrk.to/jdoe) and this mapping is known only to our system.
We plan to enhance the participants tab to display the machine recognizable identifiers associated with the referrer's short URL (http://mrk.to/jdoe <-> djeo), which would allow you to perform this lookup manually. My best guess is that we'll have this in Q2 2014.