Obviously. There was a moment an online application form was broken. myself being CS student, inspected the source code and typed `javascript:fnEditApplication` in chrome URL bar, which worked out.
Companies like Red Hat are doing it in very good way... They leave lot for the community that develops these software.
But Microsoft is obviously a cancer (It did not give anything really to FOSS ecosystem, just took from that), and Google is a double edged sword.
Google uses open source components which is highly advantageous for technical end users (Eg: I can run Linux utilities even on a non rooted Android). But their vendor lock in is second to none but Apple & MS.
I don't usually like to use such a service if there is an alternative which can be easily made offline as PDF, HTML or at best, EPUB. And better not require an account..
There are only few good ones on Wikibooks, still they are worth it... Eg: those on C Programming...
You can get some decent sources from university sites... I think I can learn things very quickly from lecture slides + bit of experimentation.
Since almost all things I want to learn are open source, there are always some great resources in form of documentation for me. I often like terse documentation rather than tutorials since it facilitates faster learning if you know how;