I've never used Scala and I'm a Ruby dev but I'm still not sure I totally agree with that. Scala is probably great and very efficient but you probably don't need to be thinking at Twitter's level out of the gate.
My side business runs off of a $20 a month VPS. My day job routinely charges millions for software that runs on one to three commodity boxes.
You'll forgive me if I don't consider "Has difficulty running Twitter" to be a knock against a language. I don't, after all, have to run Twitter.
Eventually, more servers == more headcount.
Won't every new developer who works on this codebase lose a week to two weeks becoming facile in Scala? Assume a 6 person dev team, and a conservative fully loaded $180k for a valley dev, and that's $43,000 out the window.
You can also swap in Java components when you need to so, depending on the architecture, you may only need a small Scala team. Java devs are everywhere. You also win by acquiring easy access to Java libraries.
Not saying Scala is the right choice... but there are big advantages besides "Scala is lighter than Ruby" -- and Scala is a lot lighter.
I find it honestly hilarious to hear it suggested that one of the early high-volume Rails shops should stick with "safe" language choices because they won't be able to hire enough developers to work with a new language.
I've read through the sources to Kestrel, and found them quite easy to grok. If anything, most real-world Scala code seems to be essentially very clean Java, with much of the syntactic cruft (redundant type declarations, inner classes, 'public static final' declarations, etc.) removed. Any experienced, effective developer is going to spend far more time getting to know your in-house APIs and coding style than they are the language.
Ruby is our house language. We've never hired anyone that came in the door knowing Ruby. We have a very high bar for hiring (typical first project: "write a programmable symbolic debugger for closed-source binaries on platform Y"). It takes people at least a week to become facile with Ruby --- a famously easy language to learn. Reading isn't writing.
If you have a house style, easily figure another week to pick up the right idioms, so that your code survives in the repository and isn't just refactored out.
When hiring for the small percentage of excellent programmers, I don't see language as a barrier. Sure, there will be spin up time, but you'll have that regardless. No one is experienced with every library, framework, or configuration. Adding in a new syntax might compound the issue, but I think that there is the definite potential for long-term productivity gains once the team is up to speed.
Within that niche, Rails does a fine job. For infrastructure and middleware code, however, Rails has little to offer other than the Ruby language, which has serious issues when applied to time + memory-intensive problem domains.
I'm a huge fan of Ruby, and a (perhaps somewhat begrudging) user of Rails, but that doesn't mean I think they're the right solution to every problem.
I'm saying that is false, over-optimization is what brings startups to their knees.
Google won because of good search results, but I remember that the first thing about the search engine that impressed me when I first tried it in 2000 was its responsiveness.
I just hope that other won't jump to porting their code without understanding the reasons. It should be pretty embarrassing to spend six month porting the codebase just to find out that reducing time server spends in your app code from 5% to 1% did not impact the overall speed of your site.
It would be interesting to see more details about those queue processing problems they have though.
Scala is 900x faster than Ruby for some workloads. Ruby is not a fast language. It works for web stuff because every language waits for IO as quickly as the others.