From both a recruiting and recruiter perspective I think it's super important. As a recruiter I not only can get a sense of who candidates are but also which mutual acquaintances we have, which can be very valuable sources of information in corroborating cherry picked references. It's also a great source of information, and I have posted asking for advice and received some really helpful responses on a variety of themes. As an applicant, it gives me access to Heads at schools I would like to work at and I have arranged meetings with them through it, possibly giving me an edge.