Pre-Facebook/Google+ etc online tabletop communities tended to be based around forums. Those communities are still very vibrant today and tend to be the best "hub" for activity on discussing your hobby online.
In the past I've used sites like ENWorld and RPG.net in the main really down to my RPG playing history. I still use ENWorld from time to time and rarely use RPG.net other than for their Game Index.
With other tabletop formats I've never really been part of the forum community "scene" except for geographical communities like ORC Edinburgh (obviously), Dunfermline Wargaming And Roleplay Fellowship (aka DWARF - now my Friday night club) and the Kirkcaldy Gaming Club.
With my relatively recent interest in boardgames increasing I've started to use BoardGameGeek a bit more. This is mostly for researching number of players that games can accommodate rather than reviews as I'm not a huge fan of review websites.
With the explosion of groups and communities on Facebook and Google+ as well as publisher pages etc the amount of information available is amazing; at times perhaps overwhelming when you have a cross hobby interest.
The online communities really enable you to find that specific group of people that you're looking to engage with and in many respects can help you to find other players to game with.
What online communities are you a part of and why?