Mithril is build with ERC-20, so different from Steem.
The main difference is Mithril is focusing on "Social interactions", we're trying to build more social related interactions and build up a huge ecosystem with all kind of services and products.
You can join the Mithril ecosystem simply by using the "Lit" app: http://pse.ee/5QYFZ
and start "Social Mining" there :)