Just loading a page that has the embedded Facebook Like button generates data points on you regarding your browsing and purchase history. I'm not sure if they can/do read the contents of a checkout form, but it would be simple to infer coarse location and strong identity from that.
You don't need to read the contents of a checkout form when the company just gives you the data. (Companies upload their customer lists, so Facebook can compute “conversions”. But, of course, Facebook doesn't let companies download the data Facebook has, and compare on their end… that would be a violation of privacy!)
