Steemit differs in a number of ways - firstly, as it is incentive based the type and quality of content is vastly different (depending on who you follow of course). There is more of a tendency towards longer form content and thought provoking articles written by the community. Facebook, Instagram etc are more short form content (often pictures) that search for instant gratification and likes. Traditional social media is also centralised and based on ads to make money so there is more and more ads, and irrelevant BS showing up in the feeds these days.
Also Steemit is more about making new friends as when most of us join we dont know many people on here. Although Facebook allows us to keep up with friends and family it doesnt really allow us to make any new connections.
First of all steemit is decentralized social media on the top of steem blockchain and unlike other social media which are basically centralized, it has a token economy and a reward pool to reward the content creator or authors and curators. In my opinion this is something unique that is offered by steem ecosystem.
The steem power here plays an influential role. Those who have got decent amount of steem power can productively engage, may attract good number of follower and generally seen as a respected person in this community. The steem token has a value, you can trade this token in major exchange like binance and bittrex.
When ever you create a post, it gets curated depending upon its content and how interesting it is for the audience. Good posts generally get curated and one can earn more and at the end of 7 days you will get paid. Author reward is 75 percent where as curator reward is 25 percent.
Steem ecosystem has so many sub-community developing lately and the dapps have been growing and the numbers of dapps are now such that one can easily find his own area of interest in this ecosystem. So it is not just a social media but also very precisely defined sub category of social media is also there in the form of dapps. Of course steemit is the flagship dapp. But there are other dapps also evolving around it. The notable dapps are musing. Partiko, steemhunt, fundition and many more.
Considering all of the above it is apparent that steemit as decentralised Blockchain based social media is very unique when we compare it with other traditional social media and also within decentralised social media it will lead because the features of Steem Blockchain is something which offers better scalability, zero transaction cost which are key for social blogging.
My simple answer is unlike most social media out there, steemit rewards you in cryptocurrency for doing the same things you do on other social media websites.
I find a lot more inspiration and collaboration of ideas and projects amongst the SteemIt community than any other social media site. Also, more circulation of resources (crypto and other trade) amongst the SteemIt Community that makes for fun and worthwhile engagement (contests, upvotes, businesses who take crypto, precious metal collectors/traders,etc.) than anywhere else.