In 2012 I joined InterNations. It is a community of expatriates and locals willing to share knowledge and network. There are monthly physical meetups. I was more active in the Lagos Coffee Club and attended its meetups. Made new friends, local and international. In fact, the office I use in Ikeja I got through one of the people I met and became friends with at one of the meetups.
Meetup is an online platform to join different communities based on your interest. Initially, I joined communities that hold physical meetings in Lagos. Then one day I got this excellent idea to join communities outside Nigeria. That way I can get to see what writers, programmers and entrepreneurs in USA and UK do to motivate themselves and learn from the ideas they share. And it worked. I joined all the top programming groups, top writers group and top entrepreneurs group. Every day I get to be part of the online discussion and group emails they send. I have learned a lot from the group emails and the experience, so far, as been immersive.
For my programming adventures I have found StackOverflow a lifesaver. Every time I am stuck at a program with a software project or my programming practice, I head to StackOverflow for help and I am yet to be disappointed once. There have been jobs I have done that the entire programming (code) I used were modified pieces of code from StackOverflow.
Quora prides itself as the community that provides the best answer to any question. And it lives up to that. I have learned a lot from the questions and answers in that community -- from what programming language to focus on to positive lifestyle changes to how to learn French. You will always find answers from experts in the area concerned.
And these are the online communities I have most benefited from. I would have started the list with Toastmasters International and Microsoft Most Valuable Professional (MVP) community but Toastmasters is not an online community and I am not permitted to divulge the details of what goes on in the MVP community.