It's still very common in France, at least. I see naked butts all the time as I live near the coast. Dunno about other places, tho...could be different.
As for the two pieces... I think that's the girls themselves picking those as they wanna appear more mature. Just the other day I was sitting by the changing rooms waiting for my wife and there was a mother and a grandmother with a 10-12 year old girl. They were trying to convince her to buy a one piece and she kept complaining. Dunno, just an anecdote, but it's the same stuff I hear regarding make-up and stuff as well.