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Zulip, open source Slack competitor, is relaunching after being aquired by Dropbox, Zulip 3.0. Current customers include hundreds of startups and large companies from Akamai Tech to Doctors on Demand and even Alcoholics Anonymous. It has the most active open source contributors in the space today compared to any competitor.
Zulip was sold to Dropbox while still in private beta and then Dropbox let Tim Abbot have it back after releasing it as an open source project.
Original startup's founders are Tim Abbott (Kandra Labs / Zulip founder), with Jeff Arnold, Jessica McKellar, and Waseem Daher were his cofounders. Today, they are the founders of Pilot (https://www.indexventures.com/perspectives/pilotcom-raises-40m-in-series-b-funding-and-launches-pilot-tax/).
Zulip's original 2012-2014 startup angel investors (2013) are Drew Houston (founder of Dropbox), Adam d'Angelo (Facebook's first CTO and now founder and CEO of Quora), Diane Greene (cofounder and CEO of VMWare, later CEO of Google Cloud), Paul English (founder of Kayak), Hans Robseron and Sanjit Biswas (cofounders of Meraki), Dharmesh Shah (cofounder of AngelList), Howard Lerman (founder and CEO of Yext), Enrique Salem (CEO of Symantec at the time), and SV Angel.
Zulip was then Acquired by Dropbox in 2014, while in private beta, then given back to founder. At that time Zulip became open source. Most Zulip beta customers tried switching to Slack, and then switched back to Zulip as they found it more productive (despite being a beta that was no longer developed). Dropbox decided not to pursue the team chat market, and Dropbox's legal/engineering leadership then decided to donate the software to the open source community. A dozen engineers flew to Dropbox HQ for a week to help with the engineering work of making it possible to open source Zulip. Dropbox generously donated the Zulip trademark and zulip.com/zulip.org domains.
While there are several open source projects in the team chat space, Zulip has by far more significant contributors than any of the others. Total contributors is a vanity metric; because fixing a typo via the GitHub website counts, it's inflated and doesn't give a useful picture of the breadth of a project's community. The right statistic is how many significant contributors there are - people with dozens or 100 commits ( commits are individual code contributions).
For comparison these are the other major open source team chat projects:
Mattermost: https://github.com/mattermost/mattermost-server/graphs/contributors shows 21 contributors with 100 contributions. This includes fulltime employees for their venture-funded company.
Rocket.Chat: https://github.com/RocketChat/Rocket.Chat/graphs/contributors shows 15 contributors with 100 contributions. This includes fulltime employees for their venture-funded company.
Matrix.org: https://github.com/matrix-org/synapse/graphs/contributors shows 13 contributors with 100 contributions. This includes fulltime employees for their venture-funded company.
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