Twitter is a free social networking and micro-blogging 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 as followers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications. While the service costs nothing to use, accessing it through SMS may incur phone service provider fees.

Facebook is a social networking website that is operated and privately owned by Facebook, Inc. Users can add friends and send them messages, and update their personal profiles to notify friends about themselves. Additionally, users can join networks organized by city, workplace, school, and region. The website's name stems from the colloquial name of a book given to incoming students at Zuckerberg's high school alma mater, Phillips Exeter Academy. The book shows the faces and names of the school's students and faculty.

LinkedIn (pronounced /ˈlɪŋkt.ˈɪn/) is a business-oriented social networking site founded in December 2002 and launched in May 2003 mainly used for professional networking. As of July 2009, it had more than 43 million registered users, spanning 170 industries.

Flickr is an image and video hosting website, web services suite, and online community platform. In addition to being a popular website for users to share personal photographs, the service is widely used by bloggers as a photo repository. As of June 2009, it claims to host more than 3.6 billion images, up from 3 billion in November 2008.

YouTube is a video sharing website on which users can upload and share videos. Three former PayPal employees created YouTube in February 2005. In November 2006, YouTube, LLC was bought by Google Inc. for $1.65 billion, and is now operated as a subsidiary of Google.