That's important but I think people over-estimate it. It's both. I predict that if you analyzed the github network, you'd find many hubs are based around companies that chose to move their workflows to github based on features other than network effects. Or at least, the existing network was only one of many reasons.
As a business, we (and most other companies I know) chose github for features and performance. Nice that other open source stuff is there but doesnt matter for what we pay for.
