Facebook is a social networking website launched in February 2004 and operated and privately owned by Facebook, Inc.^[1] Users can add people as friends and send them messages, and update their personal profiles to notify friends about themselves. Additionally, users can join networks organized by workplace, school, or college. The website's name stems from the colloquial name of books given to students at the start of the academic year by university administrations in the US with the intention of helping students to get to know each other better. Anyone over the age of 13 can become a Facebook user.

http://en.wikipedia.org/wiki/Facebook

Twitter is a social networking and microblogging service that enables its users to send and read messages known as tweets. Tweets are text-based posts of up to 140 characters displayed on the author's profile page and delivered to the author's subscribers who are known asfollowers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Since late 2009, users can follow lists of authors instead of following individual authors.^[4][5] All users can send and receive tweets via the Twitter website, external applications (notably including those developed for smartphones), or Short Message Service (SMS), although the availability of SMS services varies by country. The website currently has more than 100 million users worldwide.

http://en.wikipedia.org/wiki/Twitter

LinkedIn is a business-oriented social networking site. Founded in December 2002 and launched in May 2003,it is mainly used for professional networking. As of 8 April 2010, LinkedIn had more than 65 million registered users, spanning more than 200 countries and territories worldwide.The site is available in English, French, German, Italian, Portuguese and Spanish.^[4]

http://en.wikipedia.org/wiki/Linkedin

Ustream, established March 2007,^[1] is a website which consists of a network of diverse channels providing a platform for lifecasting and live video streaming of events online.^[2] The website has over 2,000,000 registered users who generate 1,500,000+ hours of live streamed content per month with over ten million unique hits per month.^[1] It received $11.1 million in Series A funding for new product development from DCM and investors Labrador Ventures and Band of Angels.

http://en.wikipedia.org/wiki/Ustream

^{Flickr is an image hosting and video hosting website, web services suite, and online community created by Ludicorp and later acquired by Yahoo!. In addition to being a popular website for users to share and embed personal photographs, the service is widely used by bloggers to host images that they embed in blogs and social media. As of October 2009, it claims to host more than 4 billion images.}

^{http://en.wikipedia.org/wiki/Flickr}