Very true.
And if Satan is ruling unbelievers (which he is) he has a kingdom.
Satan is a created being. As such, he is subject to the will of his Creator.
I see only one kingdom in scripture and that's the kingdom of God. There is a vague reference to one verse in Luke about "if Satan is against himself then his kingdom cannot stand..." but that's all. I wouldn't interpret that verse to say Satan has a kingdom.
I don't see Satan "ruling" unbelievers as much as I see him running around causing trouble. Just like the angels in heaven, Satan is limited in his abilities (e.g. he can't be in all places, all the time). Remember in Job he was roaming the earth, to and fro. In Ephesians he is seeking to devour. He is timeboxed just like the angels.
At the risk of understating it, people overstate Satan role. Really, it is my personal opinion that most of our problems are our own doing. Satan just nudges them along.