Military.com:

[T]he fact that military.com can claim “the largest online military community” with over 10 million active and retired military members is worth mentioning.  Founded in 1999 and sold to Monster Inc. in 2004 for $40 million, this monstrous site is the pinnacle in efficient social networks…. It’s one-stop-web-shopping for those in the military, where one can join a forum conversation, supplement their career with training or advice, locate a buddy, buy military gear, stay informed and enjoy oneself by just logging in.

Govloop:

With 24,000 members and a very vocal community of supporters constantly tweeting and blogging about the site, Govloop, while not the largest government social network, seems to be the most beloved government social network…. [O]ne needs to spend only seconds perusing profiles to taste the real sense of community on the site, and its sheer size and popularity make it a fed networking mainstay mainly among the Gov 2.0 crowd, communications professionals, and IT folks.

ExchangesConnect:

Launched in 2008 and now boasting over 23,000 members, the State Department’s social network aims to actualize “global diplomacy.” Given the site’s vibrant design and the motto “connect, communicate, collaborate,” it seems they’re off to a good start…. The quality of discussion on the site is very intriguing, quite possibly due to the scope of the network — they even have a book club…. It hasn’t quite reached its lofty goal, but they are well on their way to something the folks at Idealist.org would love.

GovCentral:

This one looks like a college admissions page — the staunch white and blue is more Facebook style than MySpace. The website is owned and run by Monster.com, hence the massive job board and intense career focus…. Quizzes, salary calculators, and free white paper downloads and magazine subscriptions are nice features for job seekers and those looking to kill time, but they are easily picked out as created by those who have never actually worked in government, and designed mostly for those who want to do so…. The cons are that it seems a bit more Blackberry than iPhone, a bit too scripted, tidy, and well, corporate.

Our Border:

Although the name seems to conjure up images of minute men patrolling the Mexican border in SUVs loaded with gun racks, this government social network is actually run by the U.S. Customs and Border Protection Agency…. [T]he homepage does boast an intriguing photo slide show, a timeline of upcoming events (relevant and interesting, unlike TFCN’s) including festivals, TV specials and conferences, and links to the surprisingly charming TSA blog and the less graceful Homeland Security blog.

The Federal Contractor Network:

It’s hard to say too much about this network, considering the peeving reality that every link leads you to a “sign in or sign up” page. Judging by “the Lobby,” the site’s cutely named homepage, it could very well be a useful network to join…if you’re a government contractor….To date, the social network hosts over 14,000 government contractors that must be confirmed as legit before joining. But for a social network of this size, the boards and blogs are mysteriously quiet, even with the twitter feeds streaming into the site.