While the product looks like it hasn't changed much from the outside, it's been completely rewritten once or twice, at least. When Twitter launched it was a Rub

While the product looks like it hasn't changed much from the outside, it's been completely rewritten once or twice, at least. When Twitter launched it was a Ruby on Rails app. They scaled it to the absolute limit, then started breaking services away using Scala, then they kept at it adding several other languages to the mix. While all this happened, there was almost zero product guidance. Instead, they watched for usage patterns, and started to incorporate those into the product. The original Twitter didn't have mentions, replies, or hashtags. Adding support for all that on a product not designed for it while managing exponential growth is an extremely difficult challenge. On top of that, Twitter added divisions that are known to be engineer-heavy, such as ads, search, and infrastructure. --- The comparison with Whatsapp and Instagram is interesting. Whatsapp's case is special: they famously opted to use Erlang and it turned out to be a genius decision, as that allowed them to scale beyond expectations. Whatsapp never changed much from its initial design, which means no need for deep rewrites. Instagram was a Django app running on AWS servers, and that setup was enough until they got bought by Facebook and eventually moved from AWS to FB's infrastructure.