Common Name: mixi
Niche: Japan, 18+ social networking website
What they say about themselves: I don't know, I can't read Japanese, and you can only get access to the site if you are invited by another memeber.
What I say about mixi: They must be doing something right since they have an 80% share of the Japanese social media networking market and have grown from approximately 10,000 memebers in 2004 to nearly 20 Million in 2008. I'll add that I like the idea of viral networking. Rather than "blogs" mixi calls it personal postings "diaries" (even the Japanese interface uses this word). Mixi is free to use and designed for the 18+ Japanese social networking market.

http://mixi.jp
Common Name: NetModular
Niche: SaaS - social Networking for Business
What they say about themselves: "Netmodular sites are not part of some amateur Network, we create beautiful and effective Social Networks that delight users and enhance your brand." -Jesse Tayler, CEO, Netmodular Community"
What I say about NetModular: At $9500 for setup and $10k a year, NetModular is aiming to shake a few bucks from the corporate blog wanna be's -- as Frank Zappa would say, "strictly commercial"... although I think Frank would have hired someone else if he was around for Web 2.0.

www.netmodular.com
Common Name: imeem
Niche: music - social networking / social media webiste
Slogan: What's on your playlist?
What they say about themselves: "imeem is a social network that enables users to discover, interact and express themselves with media, including music, video and photos, and form connections based on shared tastes and interests."
What I say about imeen: Nice, focused primarily on music with lots of good music. The community aspects include Battles and Pools where members vote on favorite tunes, artists and groups. Clean simple interface and not too much advertising -- if you're into music, imeen is for you.

www.imeen.com
Common Name: Hyves
Niche: Holland's #1 Social Networkng Website
Slogan: Always in touch with your friends.
What they say about themselves: "Holland's most popular website, with over 7 million members and over 2 billion pageviews per month."
What I say about Hyves.net: Well, I'm not from Holland, but the members look like they are having fun. Active user pages with lots of "friending". Videos are their own, (not YouTube). Site has a multi-lingual interface, (mostly european laguages). Main features are personal pages, videos, photos and connections to schools / classmates. The namesake "Hyves" are topical interest groups, for example their World Soccer Hyve with over 350,000 members!

www.hyves.net
Common Name: hi5
Niche: general / social networking website
What they say about themselves: "Find friends in your hometown or other parts of the world. Show off your photos. Listen to music you like. Reconnect with old classmates. Build your own page and express what's important to you."
What I say about hi5: yawn... there's nothing there. The little bit of the site you can see without logging in shows user profiles with one photo or none at all. I search for VERY popular topics like "dogs" or "pets" or "cars" and found a few dozen posts. Is hi5 still in business? If so, why?
http://www.hi5.com