Open source Twitter ? We've had that for a decade.... nobody wants to run it's own, it makes no sense, you'll have no audience. If I wanted my twitter, I'd setup a WordPress....
We kind of moved away from this monolithic setup tho. Oh you can scale-up your hardware like we used to, it will work, until some point. And the day you will need to split your services will come quickly.
This is a perfect solution for early stage projects if you're in a startup or for a personel project, but monoliths are....well....monoliths.