I really suggest you to setup a kind of viral website, CPA releated.
I mean...if your page is car releated, promote a CPA offer that allow to win a car. But before fill the offer the user must share it on their wall or, at least, a clickjacking so he'll like it and autoshare on the wall.
Your goal isn't to make it viral, but just earn extra clicks on your page.
Clickjacking thing is very easy to do, there are a lot of scripts inside the Facebook section
You probably know them...so why don't implement them?
With a 500k page a clickjacking is a must in my opinion
PS: Can your likes-man sell them targetted outside the usa/ca/uk? If yes, let me know!