If you're using a service hosted by someone else, then it doesn't matter if it's open source; you're using someone else's closed environment. In that case you're just as well off using Google or Microsoft. Probably better off, since they aren't aren't hosting on a shoestring budget.
The point to having it open is so that you can host it yourself and modify it yourself.
Not mentioned in this list — it’s unfortunate that Framasoft is shutting down many of its online and free (based on open source, of course) services in this space. It was good enough for simple needs.
That is indeed a shame and I was also not aware of them shutting down some of their services. But as far as I know their text document editing tool is a slightly improved etherpad, which is mentioned in the article.