This is just the website interface between the steemit blockchain. Every post gets mined and solidified into the steemit blockchain for as long as steemit lives on. Now the website may go down from time to time or for good for that matter , but all the website is is a window to the content recorded on the blockchain. Much like how bitcoin transactions are permanent and irreversible. If I send bitcoin to another address the tranaction block and all its info is recorded forever in bitcoin blockchain history. So a post is much like a bitcoin transaction in that way i guess. Ethereum on the other hand is reversible in some instances as we have recently witnessed.