It may seem like that, which is great - users don't need to be reminded about the backend all the time. But really, steemit is just a GUI for the steem blockchain. Steemd.com is another one, more to follow.

You can edit and delete posts, but the old data always resides on the chain and can be checked by anyone.

Account creations on steem require an inintial funding. The steemit guys pay that for new users, the Facebook/Reddit connection is just to prevent multiple signups. Anyone can create accounts with the local client and by mining.