Previously we used Google Docs. However, in every company I've worked at I've seen this tool fail. People start of using it but after a while people forget. The main issue I think is that docs lacks key capabilities in discoverability, search, and engagement; essentially confluence does a way better job of pulling our team back in so we "remember" to use it again. We have a senior manager who is addicted to the Q&A feature and looks at it daily to help answer questions from our team.
The main tools I've seen consistently fail in large orgs for documentation has been Google Docs, Google Sites, and Wikis. All of these fail in the "living" component of "living documentation". They tend to be places where you write and things die / don't get updated / are never seen.