Not an SN vs facebook matter, siggy! Information, unlike physical things, can be infinitely copied at zero cost. This is precisely Satoshi's innovation: the ability to keep ownership in an informational realm.

One might have the power to remove their own copy but not the power to remove other copies. It's just a matter of time, someone will create a bot/scraper to copy all those comments marked with @ delete. This exact same thing happened with Reddit (see https://www.reveddit.com/ ).

The only thing it achieves is make it more inconvenient for others to read older threads - the information will be around no matter how you hard you delete it.