Define advertising purpose. Rust affiliated organizations are advertising (for example conferences are being advertised).

Is only serving ads or also using ads not allowed?

Given that almost every single company in the world is using advertising then it becomes a hobbyist tool.

With serving - how about if I have a page/app that includes ads served by third party (like most of the world does). Does it make it also off limits?

Statements like that sound pretty cool, but ignore real world complexity.