For Joiners...

I may be the last one to know about this(?), but only recently discovered the MathTwitterBlogoSphere group -- it's primarily for math teachers (which is probably why I missed it, as I'm not one), but I do enjoy following their Twitter hashtag #MTBoS to see what they're up to.  If you are a math teacher, and have possibly missed their presence, worth checking out.
Learn more about them at these sites (or follow their hashtag on Twitter):

http://mathtwitterblogosphere.weebly.com/

https://exploremtbos.wordpress.com/what-this-is-all-about/

...also learn more about them with a Google search:
https://www.google.com/?gws_rd=ssl#q=mathtwitterblogosphere

Another group that formed more recently is an email group, "Popular Math," for math enthusiasts of various stripes, again to share thoughts/ideas/solutions. Check them out here:
http://popularmath.strikingly.com./

...and older standby hashtags for those on Twitter are #mathchat and #mathed (or British versions, #Mathschat and #Mathsed).