Lastly: How can developers start contributing to steemit, is it completely open-source ?

Yes! We maintain a bunch of different software packages. We have a product roadmap, so please coordinate with us in https://steemit.chat before spending a ton of time on PR to make sure it fits in with those plans.

https://github.com/steemit/condenser (this web app, js)

https://github.com/steemit/steem (the p2p consensus daemon, c++)

https://github.com/steemit/sbds (the blockchain data layer, python)

...and more. :)