Getting your code into Linux distributions, commercial unix distributions or companion packages etc is great fun but often happens without you getting to know about it, and you also rarely get any comments or feedback from those users.
Application authors or product manufacturers will also just get your stuff, build it and use it and not tell you about them using your product. This is of course perfectly within their rights. I tell you this because you could perhaps be under the impression that people would tell you these things.
How to find out that someone uses your code
The user of your project runs into an issue and asks for help in a project forum - without hiding where they come from or what product they make
While vanity-searching you find your project's Open Source license mentioned for or bundled with a product
A user emails you asking funny questions about a product you never heard of, and then you realize they found your email because that product uses your code and your license has your email