ProTeacher have very active forums to K-12 teachers for discussion, sharing, requesting assistance, and social networking. Established in 1999, its one of the most active online teacher participation sites. Topics are by grade level, subject area, jobs, centers, special ed, etc.

From site: 

The edWeb makes it easy to network and collaborate with colleagues in education outside the boundaries of classroom walls and school buildings.

• Connect with educators who share your interests
• Share lesson plans
• Compare notes on resources and products
• Discuss topics of mutual interest
• Write a blog
• Share files, images, and videos
• Message and chat for instant dialogue
• Create groups to build community and collaborate.

The Teacher Chatboards are active forums, bringing together hundreds of thousands of educators for peer support and development. There are boards for every grade level, subject area, methologies, technologies, and several hot topics.

ISTE is a nonprofit organization dedicated to improving teaching and learning by advancing the effective use of technology in K–12 education and teacher education. In their community forums, they offer information and networking opportunities to educators as they implement new technologies in their schools. Free registration is required to access the forums.