Fun fact: Twitch (which probably had the most ridiculous month-over-month growth in recent years, possibly since TwitteR) is in Rails...
And my favorite, Raverly (~7 mil user accounts), is done by two engineers (or was, not too long ago - it's hard to get numbers on these things). Two! Talk about developer efficiency.
Twitch is moving all they can to more efficient technologies such as Go. At that scale every CPU cycle counts and any delay is perceived by users.
"We use Go at Twitch for many of our busiest systems. Its simplicity, safety, performance, and readability make it a good tool for the problems we encounter with serving live video and chat to our millions of users..." - blog post a year ago.
I did my research. My research indicated that Twitch got set up in Rails, and first transitioned their chat client out of Ruby land.
I was not able to determine when Twitch began doing this, but seeing as their first major growth spurt was on launch, seems unlikely it was before they had to scale Rails to a substantial degree. I was also not able to determine what else they've switched over.
Old news is still relevant information, particularly when that news is "went from zero to ~10m on tech X".
And my favorite, Raverly (~7 mil user accounts), is done by two engineers (or was, not too long ago - it's hard to get numbers on these things). Two! Talk about developer efficiency.