Some comments both here and on another site led me to wonder: What are the largest online communities? I'll limit myself to English-speaking communities for now.
Why is this useful? Well, it shows who's doing what online, and people on the internet are probably the best people to talk to if you want to leverage "netroots". A lot of political discussion also goes on in all these forums, whether overtly political or not, and a lot of people form their opinions largely based on some of them.
And, it's also bizarrely surprising, at least to me: I'll bet you didn't realize there were forums you'd never heard of with over 1 million registered users...
I'll list the ones I know of; please add more if you know of any others!
There's some subjectivity here in what I consider a "community". Livejournal doesn't count, for example, since it's really more of a blog hosting service that happens to have a lot of separate blogs on it.
In approximate order of traffic, as counted by Alexa's traffic rankings (lower number, more traffic). To calibrate the rankings somewhat, Slashdot, the most-visited site on this list, has an estimated readership of around 4 million (source), with ranks below it dropping off in readership exponentially. Some additional data from the Big-boards.com statistics.
Oh, I also am omitting video-game communities, because there are a lot of them, and they are goddamn enormous. They'd make up the top 10 just by themselves, dwarfing anything else. IGN.com, Gamespot.com, Gamespy.com—all in the top-300 most-visited sites on the internet. Oh, specific product forums don't count either (e.g. the official Apple Computer forums).
So, on to the rest.
Slashdot
Online since: 1997
Alexa rank: 1500 (typical); 350-2200 (12-mo range)
Total registered usernames: Around 800,000
Active users: ?? (around 10,000-30,000?)
Comments per day: ~9,000
Topics: Technology
Political views: Strongly in favor of civil liberties; in favor of open source software and open, unpatented technology; opposed to most intellectual property laws; their two biggest US legislative issues are the DMCA and the Patriot Act. (Views on economic issues are across the board, from libertarians to socialists and everything in between.) There is also a separate politics section that presents fairly mixed coverage, with editors including Republicans, Democrats, and Greens.
Fark
Online since: ??
Alexa rank: 1600 (typical); 1000-2200 (12-mo range)
Total registered usernames: ??
Active users: ?? (around 5,000?)
Comments per day: ~10,000
Topics: Weird/unusual/funny news
Political views: No particular shared views
Gaia Online
Online since: ??
Alexa rank: 2000 (typical); 2000-infinity (12-mo range)
Total registered usernames: Around 1.4 million (!)
Active users: One hell of a lot
Comments per day: Too many
Topics: Anime role-playing
Additional comments: What in the world? I'd never heard until this forum until 10 minutes ago, but multiple sources confirm it's actually the largest and most active community on the internet, even eclipsing Slashdot (although Slashdot has more total readers, since there are a lot of people who read it but don't post).
FreeRepublic
Online since: ??
Alexa rank: 2000 (typical); 800-3000 (12-mo range)
Total registered usernames: ??
Active users: ??
Comments per day: ~1000
Topics: Politics
Political views: right-wing
AnandTech
Online since: 1997
Alexa rank: 2500 (typical); 1100-4000 (12-mo range)
Total registered usernames: ??
Active users: ??
Comments per day: ~10,000
Topics: Technology
Political views: No strong views. Many people with similar viewpoints to slashdot's, but less strongly and uniformly so.
SomethingAwful
Online since: 1999
Alexa rank: 4500 (typical); 3300-7500 (12-mo range)
Total registered usernames: About 55,000 ($10 fee)
Active users: Around 20,000
Comments per day: ~35,000
Topics: Irreverent humor; contributing things like "teh" and "photoshop contest" to the lingua franca of internet culture
Political views: Opposed to political correctness; in favor of cynicism and humor that offends people (On normal political issues, there's a mixture, although the average is probably left of center.) As of late, have been staging "fauxtests", with the more politically-oriented ones having picket signs like "BOWSER=HITLER" and "NO BLOOD FOR MANA", and the less politically-oriented ones including e.g. a protest against winter weather ("GOD HATES FOGS").
DailyKos
Online since: ?? (2002?)
Alexa rank: 7000 (typical); 2000-14000 (12-mo range)
Total registered usernames: About 40,000
Active users: Around 5,000 (?)
Comments per day: ~2,500 comments
Topic: Politics
Political views: Well, you know...
So that's seven... anything else major that should be up here to make it a top 10 list?