Steem - Who maintains the blockchain?
The Steem blockchain is verified by witnesses.

From the Steemit faq: The Steem blockchain requires a set of people to create blocks and uses a consensus mechanism called delegated proof of stake, or DPOS. The community elects 'witnesses' to act as the network's block producers and governance body. There are 20 full-time witnesses, producing a block every 63-second round. A 21st position is shared by the backup witnesses, who are scheduled proportionally to the amount of stake-weighted community approval they have. Witnesses are compensated with Steem Power for each block they create.

Each Steemit account can vote for up to 30 witnesses.