Meta SO is also the global Meta for general network-wide SO/SE issues which are not specific to a single Q&A within the network.
If you look at the FAQ it states:
If your question is about:
- Stack Overflow
- Stack Exchange
- Stack Overflow Careers
- Promotions & Advertising Support, feature requests, or bug reports for the core Stack Exchange engine that powers all Stack Exchange websites …
it is welcome here.
So it's not merely for questions about Stack Overflow. There will however be a separate Meta for Stack Overflow at some point.
I am adding one more point to answer of Bart.
Currently all feature request, bug reporting, Promotions & Advertising Support is done on Meta StackOverflow. Hence it is shown on all site's footer. But this will be replaced in near future with MetaStackExchange