Welcome akado!
They wouldn't have to use steemit.com but they could certainly use the Steem blockchain. Any blog, news sight, forum, etc could add their own interface to the Steem network and do whatever they wanted with the data for presentation etc.
Obviously if they wanted to change how it works underneath they'd have to start their own chain.
RE: Hi, Akado from BitSharestalk here. Got a couple of questions.