I have been trying to keep in touch with my former students, so I joined linkedin. My main goal was to find alumni I could put current students seeking jobs with in touch. Finding a job is costly (it takes time) and by developing a network these cost can be reduced. Economic research (like this paper) also suggests that by developing a network the best workers can be matched up to the right job, improving productivity of both the worker and employer. Recently I helped an employer find someone to hire I had worked with both are happy and there appear to be productivity gains on both sides. Plus I get bonus good feeling utility!
So far I think linkedin could work well. I have already found former students working for Congress and in Yemen who I hadn't been in touch with since graduation. So if you are a former student or not here is my linkedin page