Today I joined:
A professional relations web site / database which several colleagues and some friends joined already. However, I could not find any former student from my promotion, nor contact from other companies I use to work with. Maybe it will get better with time.
A US friend to whom I pointed the German network also pointed me to http://www.linkedin.com an US network. However this last one seems a bit less friend to user to establish contacts (get your Visa Card ready…)