Re: 'It's not clear whether he also has a FB account or whether he's a non-account'
Let's take the thought exercise a bit further ..
They have a bunch of data that is classed as personal. You may even go so far as being able to deanonymise some of it making it potentially identifiable (don't ask me how, but the deanon crowd can be scarily inventive when they get a hold of big datasets).
For any particular data element, they can say that they don't know who it is about. Therefore there is no way that they can evidence any informed consent for the collection and processing of said data. The individual is not (necessarily) a user of Facebook so there is no way that the data is collected as an essential part of any service provided to the individual. Therefore, as far as I can see, they would have no legal basis to keep hold of the data and should therefore delete it.
That will probably save them megabucks in storage costs ;)