Blogs are a great resource for any category or field of interest. While I haven't necessarily "built" a PLN through comments or messages left on others' blogs or posts, I follow quite a few. Leaving comments just makes me nervous. I am too proud- what if I write something that I then come back to later and think, "What the heck was I saying? That's not the way things should go down in the classroom!" I tend to be a babbler in person, so I find it somewhat amusing that I am so tight-lipped in terms of professional material on the interweb.
I have a diigo account that I have dabbled with here and there, but I haven't really built much of a network there, either. If I had more time to spend perusing and organizing the few sites that I have added to my diigo account, I'd probably utilize it more. I follow Holly and the sites she adds, and her stuff is pretty much always applicable and relevant within the English classroom (duh).
One blog that I intend to continue to follow is Quinton's. Not only should he write a book using his musings on our profession and the movement of 21st century learning, but how does he have so much time to make his look so creative and professional? Q's blog