Facebook (stylized facebook) is a social network service and website launched in February 2004 that is operated and privately owned by Facebook, Inc. As of January 2011, Facebook has more than 600 million active users.
Users may create a personal profile, add other users as friends and
exchange messages, including automatic notifications when they update
their profile. Additionally, users may join common interest user groups,
organized by workplace, school, or college, or other characteristics.
The name of the service stems from the colloquial name for the book
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. Facebook allows anyone who declares
themselves to be at least 13 years old to become a registered user of
the website.
Facebook was founded by Mark Zuckerberg with his college roommates and fellow computer science students Eduardo Saverin, Dustin Moskovitz and Chris Hughes. The website's membership was initially limited by the founders to Harvard students, but was expanded to other colleges in the Boston area, the Ivy League, and Stanford University.
It gradually added support for students at various other universities
before opening to high school students, and, finally, to anyone aged 13
and over.
Twitter is a website, owned and operated by Twitter Inc., which offers a social networking and microblogging service, enabling its users to send and read messages called tweets. Tweets are text-based posts of up to 140 characters displayed on the user's profile
page. Tweets are publicly visible by default; however, senders can
restrict message delivery to just their followers. Users may subscribe
to other users' tweets – this is known as following and subscribers are known as followers or tweeps.
All users can send and receive tweets via the Twitter website, compatible external applications (such as for smartphones), or by Short Message Service (SMS) available in certain countries. While the service is free, accessing it through SMS may incur phone service provider fees. The website is based in San Francisco, California. Twitter also has servers and offices in San Antonio, Texas; and Boston, Massachusetts. Twitter, Inc. was originally incorporated in California, but as of 2007 is incorporated in the jurisdiction of Delaware.