I subscribed to the professional network linkedin.com. It's the first time I register to this kind of website and I have to say that I found it useful for security professionals. First, you can get in touch with experts in Information Security by connecting to security groups or inviting friends of friends. I also like the questions and answers section. You can ask questions or participate to answers on high level security topics and since the people that answers the questions gets a note, often high quality answers are provided on very interesting IT Security topics.