I have used Pepsi Max, it has Citric acid but I was at work and I was exhausted and thirsty and still had to do an hour or two before getting the train home. So I did it, it made no difference and didn't knock me out of ketosis.
I wouln't make a habit of it though, and I imagine that Fanta would have a hefty amount of citric acid in it.
anything with a citric fruit flavour in it (orange/lemon etc) will have a considerable amount of citric acid... hence the name. Thats why theres so very few juices/fizzy pops that one is allowed when on Exante.