Apache Aurora, Bootstrap, Storm, Heron, Pants, ThriftMux, DistributedLog, Apache Cotton, Iago, just to name a few.
[full disclosure, I wrote one of the projects mentioned above.]
At the company I work at, various of the above projects are integral to one system or another (though to be fair, there are quite a number of ex-Twitter folks floating around). I'm not a Scala expert, but as far as I can tell Twitter also has a large footprint in that community.
(edited above post to remove Mesos mention).
His linkedIn bio: Member of the Technical Staff Twitter March 2010 – May 2013 (3 years 3 months) Built out Apache Mesos (http://mesos.apache.org).
Stop looking at everything with dev eyes.
Google, Apple, Amazon, etc., all have much more interesting products, which in turn gives their technical staff many more interesting problems to solve.
The common advice here is "let Twitter be Twitter," i.e., they don't need to expand into new products, let alone crack VR, or self-driving cars, or drone deliveries. That path will need to accompanied by major restructuring.