I am going to give several tries.
revolutionary, groundbreaking, innovatory, pioneering, avantgarde, but the best although it doesn't describe steemit but just a part of it is: FREEDOM!
I think all of these ones do a good job describing certain aspects of the platform.
Edit : I thought it said in 1 word let me try again : steemit is a decentralized, selfgoverned social media platform that is backed by a cryptocurrency or a virtual currency, where you can earn money for creating or curating content uploaded to the platform.
I think that is steemit summarized in a sentence.