It takes a while to get a billion dollar market cap. So if you use that test you're looking at old technology decisions.
A better test, for someone starting a startup now, would be to look at what's used by the companies started in the last year that seem most likely to one day have billion dollar market caps.
It's too hard to judge which apps started this year will most likely have billion dollar market caps - after all, if you'd done that in 1996, you'd think "AltaVista, Excite, Value America" and other similar nonsense.
Instead, how about looking at what companies with billion dollar market caps are buying. After all, that's the biggest statement of "We think this is really promising, but we can't build it internally, so we're going to fork over a billion dollars for it" that a big company can make.
Judging from this, the big languages seem to be Python (YouTube, Reddit) and Java (FeedBurner, Zenter). There are also a lot of recent acquirees where I haven't seen anything about their technologies - it's certainly conceivable that many of those are C++. Really, if I had to draw conclusions, it'd be "The language you use doesn't really matter - the important thing is that you build something people want."
I don't know if you're entirely correct on the old school/new school comparison. It's not as if Google decided way back in the late 90's that they'd (re-)implement Writely in Java just because it was en vogue. That's a newer technology decision than implementing reddit in python...
Ultimately, the question of what to code in is not nearly as crucial to success as we're making it out to be. And answering that question "correctly" has a lot to do with who's coding, how the code's used now, how you think it'll be used later, and many other questions.
I think it's at least worthwhile to know why other people use boring languages. Then feel free to use cool ones instead, not because you're impressed with how well Basecamp runs, but because you know you don't need Google-sized scalability right out of the box.
I suspect Google decided long ago to create a server infrastructure for programs written in certain languages, which in turn dictated how they rewrote Writely years later.
Choice of programming language may improve performance by a constant factor at best. I can't see how Google-sized scalability depends on that. I'd be happy to grow 1/10 or 1/100 of Google!
A better test, for someone starting a startup now, would be to look at what's used by the companies started in the last year that seem most likely to one day have billion dollar market caps.