Chrome was dramatically faster than anything else (thanks to v8 which was a total revolution for js engines) and each tab crashed separately, with far better security than other browsers of the time. On Windows (and initially it was only on Windows) this was a big deal and so it displaced a huge proportion of the tech aware userbase very fast, permeating out from there.

The billboards were a thing but not what drove initial adoption to critical mass.

It was remarkable how quickly Chrome spread among the employees. Nothing to do with advertising -- just that it was so much faster and the whole app never crashed.

Whenever you heard someone cursing because their browser had crashed and they lost their tabs and immediate work, someone in the next cubicle would go, "why aren't you using Chrome?"

It really was that simple. It was all word of mouth.

They were then screamed at for being insufficiently "complacent" when they tried to fix those issues by changing the extension model.