The team's own account of their growth attributes nearly all of it to Twitter: the launch tweet, monthly build-in-public update tweets, and engagement with the startup/marketer audience there.

“Social media (specifically Twitter) represents almost all of our growth”

Chris Lu's explanation for why Twitter specifically worked: their customers (startup founders and marketers) were already congregating there, so building in public in that venue put the product in front of the right audience.

“Twitter worked for us because a lot of startup founders and marketers are on Twitter”

As the company scaled past its first users, it also used Twitter as a hiring channel: announcing each new hire publicly and having the new employee post a thread about why they joined.

“Twitter has also directly brought us our first 3 hires and has played a significant role for the rest of our hires as well”

A third-party writeup (not the founders themselves) describes Copy.ai offering free weekend access to entrepreneurs as an early community-goodwill tactic. This is plausible given the freemium model but is not something Paul or Chris state directly in any founder-authored source we found, so it is included as an unverified observation rather than a confirmed play.