Yes I think the misunderstanding is that sharing a status update on Facebook constitutes sharing a file; I assumed you meant "sharing a file" like "I have a PDF I need to get to someone" and just could not imagine a world in which that constituted the majority of Facebook usage. :)
If you had said "creating an entry in a database" instead of "sharing a file" I don't think I would have had the same misunderstanding.
Sharing a file also has different connotations from uploading an image; when I upload an image to Instagram I am not doing it to share the raw file with someone so they can download it (though they certainly can if they use a browser extension to do so); I'm sharing it because that's where the people I know can see and interact with it.