So the meters falling are actually chunks of the barrier keeping the hallow and the corruption from fully entering the world, thus the short message whenever one is destroyed. However the shadow orbs are not capable of completely destroying the barrier themselves while the WoF is still there, as the wall uses it's power to seal the breaches as they come up.
The Eater of worlds is a creature of corruption that rests on the barrier and can overcome the Wall of flesh's temporary seals, but is not motivated to (except when you offer it food). When three orbs are destroyed it simply falls through (not of it's own volition so it's quite pissed about it)as the barrier becomes too weak to support it. And of course the wall of flesh reseals the opening when that happens. The eater of worlds is not part of the corruption's plans to spread, it is not intelligent, it is simply an animal native to the corruption plane.
Once the wall is defeated however the barrier is completely weakened, no longer a hindrance for the invaders even without any orbs destroyed they can still sneak in through the cracks that had cropped up over time that the wall had been fixing. The orbs themselves are dimensional bombs that they had managed to quickly propel through the barrier like a bullet, thus the deep chasms dug into the earth where they are present, the corruption surrounding it is simply the energy that had managed to travel through the opening the bullet had made before the Wall had sealed it up. Capable of spreading, but without the energy from the other side of the barrier, it lacks the power to overcome simple obstacles.
I am calling that canon until informed otherwise.