Google+

I already am a member of the following Communities:

https://plus.google.com/u/0/communities/103736876540589096694 - Maker Space Ideas
https://plus.google.com/u/0/communities/117320029229694526711 - Connected Classrooms
https://plus.google.com/u/0/communities/107107406240859233858 - (private) MBAEA Media Academy

These also sound like they would be good ones for teachers to follow:

https://plus.google.com/u/0/communities/109803947056484541234
https://plus.google.com/u/0/communities/113166595976911311283