This is a good reminder not to build anything other than a prototype in a language that you're going to regret later.
So having superior language with better out of the box speed in default implementation - I'd hardly call same place.
I think picking simple language like PHP makes it much easier to create your own fully custom stack once you need one.
Ofcourse Facebook problems will never be your problems so every one should pick what is right for him at any given moment and stop dreming about (im)possible future.
I'd say that if Facebook was java based they'd have to stick with hotspot vm. Whether it's a good thing is a completely different story.
How could Zuckerberg -- or anyone, for that matter -- have predicted the kind of rampant success Facebook has had? Who thought in 2005 that Facebook would become the largest photo sharing site on the web? Or that they would need to handle almost 140 million unique visitors per month?
If the largest problem of your business is your application stack, you are in a very good place. Most businesses have much larger problems like, say, cashflow.
That seems low. According to http://www.facebook.com/press/info.php?statistics, it's 800m active users, more than 50% of which log in on any given day.