I worked at IG on a previous iteration of Threads, with some of the engineers who wrote the current Threads app. It's Django!
(Heavily modded, run on a custom Python JIT, and using an extremely custom FB-developed database, also used for IG and FB.)
It's Django because IG was originally written in Django back in the day. FB's general approach to scaling is keep the interface generally similar, and slowly replace things behind the scenes as necessary — rather than doing big rewrites. It's seemed to work pretty well.
Ultimately the language used for the web server isn't a huge deal compared to the performance of the database, for these kinds of massive social apps. Plus, well, they do have the custom JIT — but that's very new, and when I first joined IG in 2019, we were running vanilla Python in production.
(Heavily modded, run on a custom Python JIT, and using an extremely custom FB-developed database, also used for IG and FB.)
It's Django because IG was originally written in Django back in the day. FB's general approach to scaling is keep the interface generally similar, and slowly replace things behind the scenes as necessary — rather than doing big rewrites. It's seemed to work pretty well.
Ultimately the language used for the web server isn't a huge deal compared to the performance of the database, for these kinds of massive social apps. Plus, well, they do have the custom JIT — but that's very new, and when I first joined IG in 2019, we were running vanilla Python in production.