Ouch. But do these segments come from another, PD, source? Or is it just direct theft from Facebook?

Thoughtless copy-pasta at best and plagiarism at worst.

I have very little knowledge about the actual license law, but what is the right way to do that?

Let's say there is an open source project published with a very permissive license that has one function you need. Putting the whole project as a dependency is an overkill and rewriting the function with your own variable names and indentation doesn't sound like an improvement.

Would adding a copyright notice somewhere in the repository be enough?