Giveaways @ 100th Like milestones

You can't. Facebook doesn't provide that information because it's specifically against their policy terms to reward Like's. They don't want admins to be able to incentivize Like's because it takes away from the organic (and trusted) nature of the Like's and would hurt the overall quality about them. Beside that, if you have a contest, some intrepid users will quickly find out how to Un-Like the content after they've Like'd it and otherwise game the system.

In general, Facebook doesn't provide any specific user information about user's that Like your content. JFBConnect can give you some information about the user through our email notifications if they are logged into your site, but that's the limits.

Sorry I can't help you more, but hopefully the explanation above helps,
Alex

Yes, I think that's definitely the way to go

Should you need anything else, just let me know,
Alex