It is definitely not a scam. It is a content driven social media platform based on blockchain technology and has “Steem”( a cryptocurrency) as its base. It has a wikipedia page. The transaction are on a blockchain and transparent in every form. Steemit provides a hardware wallet which protects your Steem with a private key that only you can access. It is that safe that if you happen to forget your private key/password, you won’t be able to log in again. Just keep that safe and, mostly, offline.
There are more than 500k users as of January, 2018. Many users sign up on Steemit just to invest in Steem and influence the curation on Steemit.
It is not a scam.