The biggest thing for me in establishing a Personal Learning Network is getting acclimated to the vast community of educators, professional issues and topics, and additional ideas and resources that surface along the way. Therefore, when I've searched around for online communities and networks to browse, I always have in mind the goal of exposing myself to new communities and ideas. Here are some resources I have located for my PLN:
Flipped Learning Network: Great resources, how-tos, and articles on how to create and implement a flipped classroom.
Center for Teaching Quality: This network allows you to interact more with others using the site, including posting comments.
Microsoft Educator Community: Ideas, Ideas, ideas!
Teaching Channel: Gets down to the nuts-and-bolts of utilizing innovative teaching practices and connecting the dots between those practices, objectives, and outcomes. Full of handy videos too, so it's rather easy to digest.
I'm still coming around to the idea of using Twitter. It's so easy for the app to go in its own direction, and I've found it rather invasive since I've created an account, always telling me what I'm missing and who I should follow based on my location, etc. I have to play with it more to get it to settle down and stick to the things that I'm interested in seeing. I have managed to locate some good resources there to follow, however, such as: TED-Ed, Edutopia, PBS Teachers, and Discovery Education.