Online communities of mathematics educators
Communities on General-Purpose Services
While social media services like Facebook and Twitter are not built around or for professional communities, their increasing ubiquity amongst the general public provides mathematics teachers and educators opportunities to find and interact with each other using these services.
Google+
Google+ (pronounced and sometimes written as "Google Plus") was launched in late June of 2011 and offers asynchronous following (you can follow someone without them following back), threaded conversations, public and private posting, public and private "Communities," and integration with other Google services like Blogger and Hangouts. In mathematics education, Google+ has shown to be popular with a small number of mathematics education researchers, most of whom share privately about their research, writing, and life as academics. The greatest public mathematics education activity on Google+ is found in the Mathematics Education (K-12) community, which is owned by Joshua Fisher and has more than 9500 members as of May 2015. A second, smaller, and more specialized community is the Mathematics Education Research community, owned by Reidar Mosvold.
Main article: Mathematics educators on Twitter
Communities on Specialized Services
AP Teacher Community
https://apcommunity.collegeboard.org/
Curriculum-Specific Communities
Global Math Department
http://globalmathdepartment.org/
Jerry Becker ListServ
The Math Forum
Math Education Reddit
https://www.reddit.com/r/matheducation
Mathematics Educators Stack Exchange
http://matheducators.stackexchange.com/