I work at home alone (all my colleagues are in another city far, far away), so I don't have many professional networking opportunities...but then again I don't have any professional networking needs either. It's mainly to avoid feeling isolated that I seek out other people who do the same kind of work I do. For that I joined a professional association in my field (science writing) and got in touch with the other members in my city. Conferences are of course another good way to build up professional contacts.
My personal network revolves entirely around music. I play traditional Irish music as a passion (more than a hobby but not a job), and you can find communities of Irish musicians almost everywhere, even in places like Tokyo. When I moved to Montreal I had a large and ready-made community of friends simply by virtue of attending jam sessions at pubs here in town. I know almost nobody here who isn't a musician. It's also a great way to meet people when traveling.