This deserves mentioning: The collaborating writing service SciFlow now supports Zotero. You can find instructions here and here; all you need is an account with Zotero for syncing. Like the Mendeley link they provide, fetching references from the connected (Zotero) account can be a bit sluggish if you have a large library. If you’re a student writing up a term paper or a Master thesis, you will probably not notice this. If you have a more substantial collection of references, you will notice this. A downside of the Zotero link is that it searches your complete library, including notes and extracted annotations if you have this. I would have liked a more selective sync to speed up things.
So I’m still waiting for a reference search like in Authorea or ZoteroBib. With the many export styles to choose from, SciFlow easily beats Google Docs, and it works in a limited way on a mobile phone (you can log in and edit the text, but formatting etc. are now disabled in recent versions).