Here is an interesting short paper on student/faculty relations in Facebook by a couple of researchers from Georgia Tech [pdf]
They found that contact in Facebook did not have an impact on student perception of faculty. Although, here they note that a 'ceiling' effect may be at play, where these professors are extremely well-liked regardless of their online activities. They report that a third of students were concerned that faculty were on the site, while two thirds found it acceptable.
Overall I think its more appropriate for institutions to look at how to support learning networks and professional development networks rather than to encourage staff to interact with students in general social networks. [Teachers in FaceBook]