I'm a software engineer. I work down the street from Twitter. I hear nothing but great things about their devs.
of course because you don't want to piss off your neighbors...
I always keep an eye on: http://github.com/twitter lots of awesome stuff ranging from frontend stuff to distributed services. Having 100 open source projects and judging by the rate they are putting things out they do have a good engineering team.
What exactly are those devs doing? I mean, aside from scaling to million and millions of users...which happened a while ago...it's pretty much the same thing as it's always been.
If you look at their Github, they are always coming out with great stuff. They also have their name on the biggest frontend framework out there :)
Well, twitter has "devs" then they have "Devs." They're known for their Kevin Weil-type employees and not their mumble mc. staresatfeet (because for "culture fit" they hire attractive outgoing people, not mumbles mc. staresatfeet).