To quote either pragmatic programmer or the gang of four (I can't remember which), singletons make sense when, if the singleton doesn't exist, it doesn't change

To quote either pragmatic programmer or the gang of four (I can't remember which), singletons make sense when, if the singleton doesn't exist, it doesn't change behavior through the application. So for example, setting up a logging singleton probably makes a lot more sense than adding a logging dependency through every class in your code. But conversely, if the behavior you are putting in your singleton actually has effects, and can be deemed a dependency on the code that uses it, then it makes sense to make that blatant by passing the reference into each of the dependent modules, if only for unit testing, clarity and mockability.