Questions about MathJax itself are off-topic here — ask only questions about its use on Stack Exchange. If you're making a website and have trouble with MathJax, try Stack Overflow. If you have trouble with MathJax rendering in your web browser, try Super User.
MathJax is only activated on sites where it is common to need mathematical equations, including:
- Computational Science
- Computer Science
- Electrical Engineering
- Psychology & Neuroscience
- Theoretical Computer Science
See this question for the complete list.