If you think anything on the Internet can ever be forgotten... Your going to have a bad time. Passwords, one of the most protected data types, are compiled from beaches into huge databases so that hackers can use them to try to log into website. There are literally dozens of not hundreds of those password databases on the public Internet to be downloaded, not to mention private or dark web collections. If passwords are not safe, what makes you think publicly available social media would be any different?

Even if somehow the whole federation agreed to purge all post every year, things like the Internet archive and Google cache of pages would retain the data.