1. ff_press is the motherlode for Final Fantasy fans, and the volunteers who maintain it never get enough thanks! It's an almost-daily index of new FF fanfic, fanart, icons, cosplay, and much more. They cover fandom on LJ, DW, and many other places. ff_presswatch helps them track FF activity on Livejournal, so check its profile for the names of many LJ communities. Also see their del.ici.ous archive, where they tag all the links from the newsletter so that they're searchable by game, character, ship, activity (cosplay, fanfic, icons, etc), and other categories. Here's all their bookmarks for "Communities and Websites."
2. The Final Fantasy Newbie Guide put together by heyheyrenay needs an update, but it's well-organized and worth a look. It covers most Final Fantasy fan communities on LJ up through the early days of FFXII fandom.
3. ff_fanlisting is a comm where Final Fantasy fans post their fandom profiles: which fandoms they're interested in, what kinds of fan activities they do (fanfic, cosplay, meta, etc). Add yours, or look for people with similar interests! They also have a del.ici.ous archive.
4. Search Archive of Our Own, a huge fan-created archive of fanworks that's been active for a year or so, and therefore maps to recent fandom activity.
Search for the tag community: ff_* and then browse each fic's tags for "Community: ff_somethingorother" to get the names of active Final Fantasy communities. If they're on LJ, the url will be http://[comname].livejournal.com
Or, a broad search: Community: "Final Fantasy" finds Final Fantasy fics posted to ANY community, though some of these are multi-fandom communities.
5. Embrace the power of the
Google search: site:community.livejournal.com "final fantasy" "community profile"
-- finds all community profiles on Livejournal which mention "Final Fantasy"
Google search: site:www.livejournal.com "User Profile" "Final fantasy"
-- finds all LJ User Profiles with the words "Final Fantasy" somewhere on the page. You may alter the phrase "Final Fantasy" to your favorite game, ship, or character, or add on another phrase in quotes.