Is Heart of Darkness the story of society's corruption and exploitation or personal corruption and greed?
I don't like the way that your question seems to force you into making a choice between these two options. Actually, I would say that the overwhelming message of this excellent novella indicates that both answers are correct. The story focuses just as much on the way that colonialism as a whole functions to exploit and corrupt as it also looks at the individual in such an arena and examines how character is decayed and eated away by occupying such an arena without law and order to restrain the darker instincts of man.
To examine the way that society as a whole is shown to be corrupt and exploitative, consider Marlow's description of colonialism at the beginning of the story:
It was just robbery with violence, aggravated murder on a great scale, and men going at it blind--as is very proper for those who tackle a darkness. The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much.
Clearly, describing the entire colonial enterprise in these terms establishes the way that it was based on society's corruption and exploitation. Yet, at the same time, equally the story focuses on the way that personal corruption and greed can result in the downfall of noble characters, namely Kurtz. Note how this is symbolised through the way that Kurtz chose to have his hut decorated:
Then I went carefully from post to post with my glass, and I saw my mistake. these round knowbs were not ornamental but symbolic; they were expressive and puzzling, striking and disturbing--food for thought and also for vultures if there had been any looking down from the sky; but at all events for such ants as were industrious enough to ascend the pole. They would have been even more impressive, those heads on the stakes, if their faces had not been turned to the house.
Kurtz's moral corruption is indicated through the abominable crimes he committed in his quest for gaining ever greater quantities of ivory and the way that this hunger conquered his moral scruples. This novel therefore clearly indicates the way in which colonialism features the corruption of society and individuals.