The two biggest FPV communities that I found and actively use are the following:
The most information, relavant to FPV flying (at least for me) can be found on these. I have created the multirotor branch on the hungarian forum a while ago and I am even moderating the underlaying topics there.
There are some rare cases, when the needed information can not be obtained from these (special projects, rare solutions). When this happens nothing is lost, as there are many other useful sites that may provide the necessary info. The best of these are EU sites, mostly german forums, but I have already found rare information on spanish, turkish, and israeli sites as well.
Apart from those, I have to mention the other big sites with FPV content:
There are also many forums directly connected to big sites of vendors that sell equipment (Fatshark, Gopro, TBS,…), if searching for a product specific information, it is rather useful to visit these forums first.