An electronic medium that allows instantaneous communication internally and externally to thousands of people through a computer server is called ______.

The correct choice is social media. Social media platforms are specifically designed to facilitate instantaneous communication with large audiences, both internally within organizations and externally to the general public. They provide a space for sharing information, ideas, and content in real-time, enabling users to interact with hundreds or thousands of people simultaneously.

Text messaging and email are forms of communication directed at specific individuals or small groups rather than open platforms for mass communication. An intranet, on the other hand, is an internal network used within organizations for private communication and information sharing among employees, which limits external access. Thus, social media stands out as the most fitting option for the described functionality of reaching a vast number of people instantly through a computer server.