Facebook.com: Was initially intended for college students -- it branched out, and now allows everyone membership. 150 million members.

MySpace.com: 191 million members. This site is massive, boasting the largest membership of any social networking site on the Internet.

Linkedin.com: 12.5 million members -- a powerful tool for business networking.

Friendster.com: 29 million members. Friendster was considered the top online social networking service until around April 2004, when it was overtaken by MySpace. Demographic studies indicate users are from 17 to 30 years old.

Stumbleupon.com: Boasting 2.75 million users, StumbleUpon is a web browser plugin that allows its users to discover and rate webpages, photos, videos, and news articles. A great way to get website promotion. Bought by eBay for $75 million in May 2007.

del.icio.us: The website del.icio.us (pronounced as "delicious") is a social bookmarking web service for storing, sharing, and discovering web bookmarks. The site was founded by Joshua Schachter in late 2003, and is now part of Yahoo!.

digg.com: Digg is a website made for people to discover and share content from anywhere on the Internet, by submitting links and stories, and voting and commenting on submitted links and stories, in a social and democratic spirit.

Orkut.com: Orkut is an Internet social networking service run by Google and named after its creator, Google employee Orkut Büyükkökten. It claims to be designed to help users meet new friends and maintain existing relationships. Now has a membership of 57 million.

Twitter.com: A free social networking service that allows users to send "updates" (text-based posts that are up to 140 characters long) via SMS, instant messaging, email, the Twitter website, or an application such as Twitterrific. The site has become very popular in only a few months -- a lot of people are watching it.

Classmates.com: 40 million members. One of the oldest social networking sites around, Classmates was kicked off in 1995, and has proven to be a great way for members to to connect with old friends and acquaintances from throughout their lives.

Meetup.com: 2 million members. Meetup.com is an online social networking portal that facilitates offline group meetings in various localities around the world. Meetup allows members to find and join groups unified by a common interest, such as politics, books, games, movies, health, pets, careers or hobbies.

Yahoo! 360° (a.k.a Yahoo! Days) is a personal communication portal similar to orkut and MySpace -- it is currently in the beta-testing phase. It integrates features of social networking, blogging and photo sharing sites.

Xanga.com: 40 million members. Xanga is a free Web-based service that hosts weblogs, photoblogs, videoblogs, audioblogs, and social networking profiles.

Care2.com: 7.2 million members. Care2 is a social networking website that was founded to help connect activists from around the world.

Ryze.com: .25 million members. Ryze.com is a free social networking website designed to link business professionals.

In all, there are over 100 social networking sites on the Internet. Some of the other social networking sites that I have not included in the list above are:

Bebo, BlackPlanet.com, Flickr.com, Reunion.com, aSmallWorld, Bebo, BlackPlanet.com, Blue Dot, Bolt, Broadcaster.com, Buzznet, CarDomain, Consumating, Couchsurfing, Cyworld, Dandelife, DeadJournal, DontStayIn, Doostang, Ecademy, eSPIN, Faceparty, Flickr, Flirtomatic, Fotki, Friends Reunited, Gaia Online, Geni.com, GoPets, Graduates.com, Grono.net, Hyves, imeem, Infield Parking, IRC-Galleria, iWiW, Joga, Bonito, Last.fm, LibraryThing, LiveJournal, LunarStorm, MEETin, MiGente.com, Mixi, MOG, Multiply, My Opera Community, myYearbook, Netlog, Nexopia, OUTeverywhere, Passado, Piczo, Playahead, ProfileHeaven, Pownce, RateItAll, Reunion.com, Searchles, Sconex, Shelfari, Soundpedia, Sportsvite, Studivz, TagWorld, TakingITGlobal, The Doll Palace, The Student Center, Threadless, TravBuddy.com, Travellerspoint, Tribe.net, Vampire Freaks, Vox, WAYN, WebBiographies, Windows Live Spaces, Woophy, XING, Xuqa, Yelp, Zaadz, Zooomr