node.js mongojs findOne callback returning error as null
When the findOne query doesn't find at least one matching document, the second parameter of the callback (in this case user) is set to null. It's not an error, so err is also null. So what you're seeing is the expected no-match-found response.
Update
Note that findOne has been deprecated in the 2.0 driver, but its replacement also exhibits this same behavior:
users.find({'fb_id' : fbUserMetadata.id}).limit(1).next(err, doc) { // doc is null if a matching document wasn't found});