The final sentence of the book is very telling. It shows that in the end the communist system was just as bad as the system it replaced.
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
Symbolically, the pigs have become the people. They act the same as the people. They are just as abusive, and exploit the animals just as the pigs did.
When the animals overthrew the humans, they did so with the intention of creating a farm run by animals. All animals were supposed to be equal, and share the work and fruits of labor. The pigs managed to take over little by little.
Two sentences before, the narrator states:
Twelve voices were shouting in anger, and they were all alike.
In other words, the pigs sound and look exactly like humans. They act like humans, and things will be no different on the farm under the pigs than they were under the humans.