The Wikipedia article on GitHub claims >1m users (passing the mark in 2011), with around 90,000 unique repositories out of 2m. StackOverflow has a few relevant questions like http://stackoverflow.com/questions/453880/how-many-developers-are-there-in-the-world which give me a vague estimate of perhaps 5-15 million worldwide. 0.1m unique repos to 15m developers is 6.6% and roughly consistent with 5%.

On the other hand, I don't know what 'personally published software' might be. A complete standalone executable or library? I could see most programmers confining their efforts to working on existing codebases, yes, and this would likely cut down the GitHub repo estimate a lot since many of them are no doubt forks with a patch or two of some main repo or dead ends of various kinds.