Then you should really know better than to make statements like "I think I could write a scalable Twitter in mostly Ruby. And I've never written a line of Ruby in my life."
This isn't really a comment on Java, Ruby, Twitter, Google, or my programming ability. It's a comment about the fact that once you've designed a scalable distributed system, the constant factor runtime isn't so important.
Twitter has clearly engineered a working distributed system (or you would have read stories like "5% of Twitter users lost their tweets when our servers caught on fire last night"), so the fact that they wrote it in Scala or Java is largely irrelevant. They need fewer computers than they would have if they wrote it in Ruby again, and that's not something to discount, but I think it's technically possible to have a full Ruby stack do what Twitter does. And, they would have needed even fewer computers if they used C or C++. So it's clear that Java was a social choice (it had the right books, libraries, and stack of resumes) rather than an absolute must.
Besides, Ruby and Java both make the same C library calls to do I/O. So it all ends up being the same for everything except search, trending tweets, and other cpu-intensive analysis.
I think I could write a scalable Twitter using mostly low-paid workers who manually write down the tweets and deliver them in person. It's just a matter of hiring enough workers now that I've designed this scalable distributed system.
Compared to the OP, your post is just random, non-funny sarcasm. The OP has highlighted relevant data points.
1. Writing a system the second time is easier since you already know the pitfalls and avoid them.
2. Ruby, Python, Perl, Java... provide thin wrappers for the underlying IO system calls, and hence, Java IO isn't very different from Ruby IO when it comes to performance.
3. There are only a few CPU intensive tasks. It goes without saying that Java will kick Ruby's(pure Ruby; native extensions will be a different story)ass when it comes to CPU intensive tasks.
Computers are easier (though perhaps less fun) to make than humans. Ruby is slow but it's close enough to Java that the numbers will still work out. Humans: probably not.