I find face-to-face networks and professional organisations far more useful than online networks. It is easier to have a conversation face-to-face, although distance might make it rarer. I am a member of CILIP and my membership of the Health Libraries Group has been especially valuable. I have learned a great deal from the professional publications, and even published myself on a couple of occasions. The conferences and other training events I have attended with them have always been excellent, and presenting myself has raised my professional profile. No new jobs though!
I am also a member of EAHIL (European Association for Health Information and Libraries). Their journal is very interesting, but I haven’t been to any of their conferences because of the cost. I am more of a lurker in this group.
Most of my networking has been with other local and regional NHS Librarians. This kind of networking often has concrete results in the form of joint projects. Whether the latter will survive changes to Deaneries as part of the NHS reforms remains to be seen.