Note that the Facebook UserId from the Facebook plugin no longer matches the Facebook user id.

Since the Graph 2.0 API was introduced by Facebook, you are no longer going to get new user's Facebook User ID, instead Facebook will give you the "app-scoped user id". It is still unique, but in most cases it is not possible to work backwards and get the original user id based on an app-scoped user id.

The Scirra Facebook plugin is fetching the app-scoped id and using the javascript parseFloat to convert the value. Unfortunately, this seems to very occasionally introduce decimal values to the id, for example, when testing with my Facebook account, the app-scoped id changed from 38742387293 to 38742387293.0005. [As of June 2014]