I don't really post much in the forums at all, but I thought I would provide my two cents on the subject of seeking out votes outside of this website.
It's a simple thought, if there had been no cash prize, would you have bothered posting to social media/other sites for votes? I think you know the answer is no. So if the answer is no, then you probably know, that doing so wasn't the fair thing to do. Whether or not it was explicitly stated in some arbitrary rules. It's common sense.
My suggestion would be to divide the top prize cash amount among the top 3 winners, and then just divvy up the remainder to the other finalists. The contest wasn't advertised on other websites, so the contestants should have followed suit.
Just my 2 cents.