For what it's worth Facebook and Instagram (also owned by FB, but is fairly separate product-wise) have pretty good export tools. You make a request in the web UI and a short time later, can download a zip with a bunch of JSON files. I was pleasantly surprised by how much they included.
I think, but I'm not sure how accessible it has to be. I was mostly commenting on how approachable the data format was. Formatted json with descriptive keys.
I have no idea what the law requires about the data format, so they could be doing the absolute minimum.