People actually swapped from horde to alliance at the dawn of p5. The server pop was 55 / 45, the majority being alliance.
P6 of classic, Alliance dominated Bigglesworth. Unfortunately, Blizzard locked out Argentina accounts in April, which locked out many people from their alliance Argentina account.
According to ironforge gg, alliance are at 30% now on bigglesworth.
I mean, that number is only people who log raids. It might not be inaccurate to suspect that fewer than 30% of the population actually raid, and among them fewer than 100% are represented in the logs.