Well keep in mind say with your social Media App you could have a plugin to facilitate connecting to your Discourse Forum.
There are already a variety of plugins created to use things like:
- Facebook Authentication
- Steam
And more. Problem with modifying/forking is you end up with potential security and Bug issues that you and your team would need to correct yourselves.
The Discourse Team has made it very customizable through plugins, Theme & Theme Components along with using css and scripts.
I would more reccommend exploring what Discourse has to offer to see how it can fit without drastic direct modification.
There is even a Social Chat Plugin called Babblechat.