If you'd tell them, that you can take a picture of a random person and search that picture out of a pool of people and get their contact info (as a private company / random programmer with enough data), they won't believe you or tell you that something like this is only possible in movies.
Even today it's no problem to analyze the Facebook picture of some random person and calculate a chance of that person being an alcoholic in X years based on the number of party pictures they share.
Before sharing personal information online, people should not ask themselves what could be done with that data now, but what is possible with that data in the future.
If Facebook is going out of business (just as a thought), their data has a lot of value. Don't think that they will delete everything. It will go to the highest bidder!