Twitter was a realtime messaging platform (fundamentally not edge-cacheable) that evolved from that CMS foundation. So the reason for the difficult evolution should be clear.
It's not really a coincidence that before Twitter I worked at CNET on Urbanbaby.com which was also a realtime, threaded, short message web chat implemented in Rails.
Anyway the point is: use my new project/company https://fauna.com/ to scale your operational data. :-)
EDIT: either it was edited above, or I’m blind. I think it was the earlier?