[First engineer at Zulip; work at Dropbox]
Portions of Zulip's backend were used to power some Dropbox features in 2014-16, but the product itself wasn't further integrated.
In any case, I'm proud of how we handled the handoff to OSS and a sustainable independent stewardship org. I have https://ourincrediblejourney.tumblr.com/post/146708555778/zu... framed on my wall.