Very straightforward traditional LAMP with lots of database sharding and various smart optimizations to handle extreme traffic volumes.

They used to have a custom file storage back-end which resembles something like swift. That may have been replaced with AWS after I left.

They've never made a secret of it being LAMP throughout. There's still some code in there from the original PHP4 site 14 years ago (or at least, there was at the time I left staff).