One of the main points of the article is that the only way to do Facebook-scale sites is to have an architecture that scales out horizontally without incurring any extra overhead, and that rails does so out of the box. So yes, you could quite easily run something at Facebook scale with rails, it would just require more physical servers than a more performant framework.
you can always solve any problem throwing money at it. I guess the main point is running the same thing with 30% less servers, not more.
You can't actually solve a lot of problems by throwing money at them. Ensuring that scaling your core application to arbitrary traffic sizes is in the class of money-solvable problems isn't trivial and should guide your architectural decisions from day 1.
Yes, you'll probably end up doing significant rewrites if you do actually win the lottery and get big, but it's nice to know that you're not going to top out at some arbitrary size and have to start turning customers away while you do those rewrites.
That said, ruby is an incredibly easy language to program in. If you start with ruby and an SOA, it doesn't matter where you start to hit scaling issues - you can replace the code (that you wrote far faster than you would have in Java) with something more performant.