http://www.opencolleges.edu.au/informed/features/20-tips-for-creating-a-professional-learning-network/

This site not only gives tips, but also mentions quite a few useful learning communities to join.

https://www.youtube.com/watch?v=vWP8b9vxXX4

Great video for visual learners explaining how teachers can build a PLN.

https://www.edutopia.org/blog/twitter-expanding-pln

This site goes into a little bit more depth on using Twitter Chats to grow your PLN.

https://www.mindtools.com/pages/article/linkedin.htm

LinkedIn, the business-oriented social networking website isn't just used for keeping in touch with business associates, clients, and former colleagues. In this article, they explore how to use LinkedIn for personal, professional, and organizational development, including how you can use it for networking and recruitment, and for keeping up with trends and news in your industry.

http://www.onlinecollege.org/2012/09/16/top-25-twitter-tips-your-professional-development/

Very organized and concise list explaining how to use Twitter for professional development.