Social media are websites and applications (apps) for content sharing and collaboration among communities of users online. The term encompasses social networks, microblogs, open-content repositories, photo or video sharing, messaging apps, social gaming, and virtual worlds among diverse other offerings. They represent a leading means of communication in the 21st century. Some of the best known social media sites are Facebook, Instagram, Twitter, Tumblr, and Snapchat. Facebook alone has about 1.6 billion active users and Instagram has about 20 billion images. See also: Digital photography; Internet; Virtual reality; Web 2.0 technologies; World Wide Web

Social media platforms. (Worldpictafolio/Getty Images)

New social media platforms are constantly being introduced and existing ones are constantly evolving, which makes defining them or generalizing about them difficult. Broadly speaking, however, members of a social media digital platform elect to form one-way or two-way communication links among one another. Members thus each become the hub of their own social network, and they can choose to share messages or other digital content with some or all of those (often designated as “friends” or “followers”) in their network. Those receiving the content may be encouraged to “like” it in response, to comment in reply to it, or to share it with their own network. In some social media, businesses may also have some access to members’ networks in the interest of advertising, promotion, or expanding their own audience. In contrast with traditional broadcasting or publishing media, the social media experience tends to be interactive and personalized, and it is often optimized for mobile users to encourage frequent interaction through smartphones. Popular content that is said to be “viral” because of how readily members share it can propagate throughout a social media platform extremely rapidly. Content may also be tagged (with hashtag text or other indexed flags) to aid its discovery in search. At their best, therefore, social media represent a fast, inexpensive way to spread a message to a widely distributed audience. See also: Cellphone cameras; Electronic communication and identity development; Location-based decision support; Persuasive technology

The influence of digital social media on societies around the world has been profound. Social media have affected both how and how often individuals stay in contact with friends and family members. They gave a louder voice to many people whose messages had historically been underrepresented in media because of racism, sexism, and other forms of bigotry. Yet social media also enabled the phenomenon of “cyberbullying” as a new form of oppression. As social media rose in prominence, they powerfully disrupted the business models for most newspapers, magazines, broadcasters, and other traditional media and hastened the failure or transformation of many of them. Epidemiologists, sociologists, and informatics researchers often now use data mined from social media to explore the spread of diseases, ideas, and trends through populations. See also: Big data; Data mining; Data mining process; Epidemiology; Mining Internet search and social media for epidemiology; Personal chemical exposure informatics