I used to work there and indeed the source code is a huge monolithic PHP nightmare based on an ancient version of some forgotten framework and hacked on by hundreds of developers over the years. There was a huge amount of code dedicated to rendering content that depended on bugs from the application’s earliest days plus new behaviors that were introduced over time. Every request required a full start of the entire tumblr application - they didn’t trust the code to process more then one request per invocation so it exited after sending a reply. Developer turnover was huge due to the code base being a nightmare and no management support for refactoring.

But if you could stomach working with the code the catered lunches from Dos Toros and Luke’s Lobster were epic.