Facebook has already done all the hard work for you. They asked when people were born the day they set up their accounts.  

They did that so marketers like you; the smart marketers, can make them an offer they can't refuse. After all, it's their birthday. 

If the lifetime value of your customers is worth the investment you can consider offering a coupon that provides a free gift. 

The fewer conditions you attach to the gift the more likely you are to earn a customer for life but that doesn't mean just offering a great deal won't win you customers, as well.