The most important Social media sites:

Facebook - Facebook is the world’s largest social network, with over 1.15 billion registered users.

Twitter -  Estimated registered users 500+ million.

Google+ - or Google Plus – is a social network from Google.  Estimated registered users 540+ million.

LinkedIn – Founded on May 5, 2003, by Reid Hoffman and founding team members from PayPal and Socialnet.com, LinkedIn connects the world’s professionals to make them more productive and successful.  Estimated registered users – 259 million.

Foursquare- The idea here is to download the mobile app for iPhone or Android and check-in to venues that you visit.

YouTube – YouTube was created by Chad Hurley, Steve Chen, and Jawed Karim in 2005.
Pinterest -  Estimated Unique Monthly Visitors-140,500,000

Tumblr – Tumblr lets you effortlessly share anything.Post text, photos, quotes, links, music, and videos from your browser, phone, desktop, email or wherever you happen to be.  125,000,000 – Estimated Unique Monthly Visitors.
StumbleUpon -StumbleUpon (SU) was originally founded in 2002 by Geoff Smith, Garrett Camp and Justin LaFrance and is now owned by eBay..Estimated users 20,000,000.

Instagram – Instagram has grown to be one of the most popular social networks for photo sharing that the mobile web has ever seen.  Estimated users 100+ million.

VK(Originally VKontakte)  -VKontakte is Russia’s Facebook and the largest independent medium in the country. The founder of VKontakte, Pavel Durov, had long resisted all pressures to step down. 65,400,000 – Estimated Unique Monthly Visitors

MySpace – MySpace is a popular social networking website offering an interactive, user-submitted network of friends, personal profiles, blogs, groups, photos, music and videos internationally.  Estimated Unique Monthly Visitors – 26,500,000 . 

Flickr -Yahoo’s photo sharing service Flickr is almost the best online photo management and sharing service on the Internet.

Empire Avenue – Empire Avenue allows individuals and businesses to measure and benefit from their online influence across blogs and social networks like Facebook, Twitter, Flickr and more.

Bebo:- Launched in July 2005, Bebo has steadily risen to become one of the world’s most popular social networking sites.In the United Kingdom, Bebo is the second best social network. Estimate users 117,000,000..

Friendster: -Friendster was one of the first Web sites to bring it into mass culture. Started in 2002 by U.S. businessman Jonathan Abrams.  

Badoo:- Badoo is a social network where you can meet new people. With over 180 million users, Badoo is great for chatting, making friends, sharing interests, and even dating!  Badoo includes geographic proximity feature that identifies users’ locations based on analysis of their network connection.

Mixi:-  was launched in February 2004, it’s the biggest social networking site in Japan.

Ning:- Ning is a company co-founded by Marc Andreessen (of Netscape fame) and Gina Bianchini. It launched its service in October 2005.

Last.fm:- Last.fm is the flagship product from the team that designed the Audioscrobbler music engine.

LINE – LINE is a new communication app that allows you to make FREE voice calls and send FREE messages whenever and wherever you are, 24 hours a day! Is a Japanese proprietary application for instant messaging on smartphones and PCs. LINE has more than 380 million users worldwide and is used in over 231 countries!
Classmates: Classmates.com profile is free and easy. Classmates is different from most social networks, in the sense that most of its features are available to premium member.Classmates is primarily used to reconnect with old classmates.

Reddit – Reddit has a very strong and closely knit community.
Flixster: Flixster is a social networking site for movie fans. Users can create their own profiles, invite friends, rate movies and actors, and post movie reviews as well.

Meetup: Meetup is an online social networking portal that facilitates offline group meetings in various localities around the world. It makes it easy for anyone to organize a local group or find one of the thousands already meeting up face-to-face.

European Social Network- The European Social Network (ESN) brings together directors of social services, regions, provinces, counties and municipalities, universities and research bodies from over 30 countries to network and learn from each other, share good practices and influence European and national policy.