Facebook (and others) know that giving you that feeling of ownership and customization keeps you coming back. The Twitter Avatar, the Facebook profile, the MySpace page. All those posts on your profile add to your collection, your customization. They let you control that little bit so that you feel like you own it in some way. Did you love that old car so much that you put a bumper sticker on it, or did putting a bumper sticker on it make it more "yours" and so you loved it more? Same idea.
But you don't own it. Nothing you post on those platforms belongs to you anymore. It's theirs, to do with as they wish. And in this case, Facebook decided that retroactively deleting all that content that lets you be aware of other platforms was good for their bottom line- do they did what they wanted with their property.
Facebook deleted their posts. You only thought they were yours because Facebook makes more money if you think that.