It is probably fair that a huge share of code that is Foss is licensed under GPL, much larger than the share of source available proprietary licensed code
MIT is used by more projects than GPL.
Basically all GPL code is available on the web and there is a vast amount of it. I barely see any current non-FOSS code on the internet, although I think it would be fair to count the big projects who have been using pseudo-OSS licenses lately as proprietary. Wouldn't a safer assumption be a ratio of 10:1 or 100:1 for lines of GPL vs. lines of "shared source?"
Basically true if you add "to paying customers", there's no obligation to publish it otherwise. You can even sell your GPL software on DVD if you like.