When I moved to California, I used the term “pop” to refer to carbonated soft drinks (such as Coca-Cola and Pepsi). I assume that’s because it was the most popular term used where I was coming from. Very few people knew what I meant, except other people from the same region or the mid-west.
After a few years, I adapted and began calling it “soda.” Now, most of the time I just order by name (if there’s a choice on a menu, etc.) and just say “soda” or “drink” if it’s one of those self-serve fountain machines.
StrangeMaps blog has a little discussion started on this interesting map. The “Soda vs Pop” web site has compiled the usage of these terms into a map based on individual respondents to their survey by county, which can be viewed on the Soda vs Pop web site.