The term tribe is used as a slang term for an unofficial community of people who share a common interest, and usually who are loosely affiliated with each other through social media or other internet mechanisms. The term is related to "tribe," which traditionally refers to people closely associated in both geography and genealogy.
The concept is closely related to social networking, and dates back to at least 2003, when Tribe.net was launched. Cory Doctorow wrote a science fiction novel that expounds on this concept released in 2004 called Eastern Standard Tribe.