Steemit is fundamentally different than FB and Twitter. It runs on a blockchain. Every user action on Steemit is recorded on the Steem blockchain. Writing these records uses real resources provided by entities called Witnesses ... that is... people who runs servers that generate Steem blockchain hash blocks for use in recording information.
Steem Coin is an application running on the Steem Blockchain. The reward system is used to pay to run the system. SP is comparable to stock purchases in a regular company.
Here's a decent post on RC ......
Resteemed this thread for more visibility... sharing what little visibility I have.