I used to hang out in the IRC channels ##c++ and ##c++-basic at freenode but that was many years ago. Note that these channels were, and probably still are, very strict and not a good place for socializing about unrelated topics.
I often read, but has never participated, in the ISO C++ google groups. (https://isocpp.org/std/forums). It contains a lot interesting discussion about new proposals for C++.
I sometimes participate in discussions about C++ on YouTube (even though their comment system is terrible). There are a lot of old, bad, misleading videos about C++ and many confused people to "help". ;) There are also some very good videos. I really enjoy watching videos from CppCon (https://youtube.com/user/CppCon/videos). It's both entertaining and something that I can learn a few things from.