I'm aware that they existed, but clearly they were very immature as compared to how they are now. PHP on the other hand was in it's heyday, and (clearly) fulfil

I'm aware that they existed, but clearly they were very immature as compared to how they are now. PHP on the other hand was in it's heyday, and (clearly) fulfilled all of the needs of Zuckerberg and the other developers. A suggestion at the time that they use a language like ruby or python would probably never have been made, and for good reason - and it almost certainly wouldn't have been adopted. It's just not realistic to suggest that early FB had this vast array of choices for which language to use.