On Facebook's "Download your information", it says "Download a copy of your information to keep, or to transfer to another service". Is there another service to transfer to? Maybe something using ActivityPub like Mastodon? I can't find one.

There are third party apps that allow you to post to Facebook and check your Facebook status. Couldn't that other service post to Facebook and Mastodon on your behalf and read your friends profiles? I can't think of a reason it isn't technically possible for something like that to be created, but I haven't seen it created yet.