The one I use the most is LinkedIn. It's like a virtual rolodex. I love the ease with which I can update my own information. If I need to change my phone number or other important contact information, I simply do it on LinkedIn and my contacts can access it immediately. I like the privacy settings that I'm allowed to set.
It's also a bit like playing "Six degrees of seperation". I've been amazed at how some of my seemingly unrelated connections know each other, which is also great for networking and introductions.
If you're in any international education field -- or have an interest in getting involved, you might be interested in joining the LinkedIn group called Learning Abroad Professionals (LAP).
Unlike some professional organizations that have LinkedIn groups -- but require membership in the organization -- LAP is open to anyone interested in the field and can help you connect globally with all sorts of professionals in international educational exchange.
There are opportunities for posting questions and being part of discussions. And there are even jobs/positions posted that don't appear on other listservs.
You're welcome to join Learning Abroad Professionals (LAP)!