I've discussed it on some forums now and didn't get the response I expected at all: people seem to think it is an obscure term that is used only in the South (which I've never even visited) and/or that no one under the age of 60 uses it.
So my question to the community is this:
1. Do you know the term "fountain drink", "soda fountain," and similar terms? Would you know what someone was asking for if they ordered it a fountain drink?
2. Where are you from/where have you heard it?
3. How old are you? (Or age range, whatever if you're not comfortable answering this)
I had never thought about it before, but now I'm very interested to find out where the term is common and where it isn't. I guess I'd assumed it was common because the term "soda fountain" was universally used, but perhaps it isn't and I never knew!