Steemit.com is just a website. It is hosted on a server(s), and can totally be shut down. Steem is a cryptocurrency that is produced on a blockchain. The blockchain is decentralized, and while the POS VP weighting scheme leaves the Steem blockchain vulnerable to Sybil attacks that are extremely easy, if expensive, the blockchain is relatively immune to being shut down.

Steemit, Steem, and the blockchain, are different things.