On the other hand, as Git is a DVCS even if GitHub went down the repos still live on local developer machines, so it'd be easy enough to move to a different Git hosting provider.
People keep bringing that up, but that's missing the bigger picture: GitHub is more than just a git host, it's also a "social hub" where people can fork repos and offer changes back (pull requests), a passable bugtracker (issues), even wiki, and even web host (github.io).
Many people have warned that using these added-value features makes you dependent on GitHub (Linus Torvalds has a kernel mirror on GitHub, but for this reason refuses to use its other features). Migrating all that metadata is hard (is it even possible?).
Joey Hess' github-backup backs up everything github knows about a repository (including branches, tags, other forks, issues, comments, wikis, milestones, pull requests, watchers, and stars) to the repository.
I'm sure most of these could be re-imported into an alternative service through its API, even if with some loss of fidelity.
(for others: he's also the creator of git-annex, an out-of-band file storage extension to git, and for a long time was the maintainer of critical parts of Debian's ecosystem. He likes to live in a yurt: http://joeyh.name/yurt/)
I think Github is a prime example of how much a "social hub" can add. In the 6 years or so before GH, do you think I ever send a patch upstream? Now, collaborating is almost easier than not doing it, and it's an awesome community.
Oh, but they do add a lot of real value. In fact, they're extremely efficient at making this value - so efficient that they don't get to monetize anything but a small fraction of it.
I moved a bunch of repos to bitbucket, since they support free private repositories (which I need for a small organization that I'm part of). It was, as your comment alludes, extremely quick and easy thanks to the "D" in DVCS.
It's worth nothing that GitHub isn't just a pretty Git frontend anymore. It's also a de-facto CDN and host for projects (Homebrew, CocoaPods come to mind), as well as a blogging and demonstration platform (GitHub pages).
In these roles, GitHub is of tremendous value and not immediately replaceable.
It's my impression that they really don't want to be a CDN. One might suggest that as a service that could generate revenue if they charged for it, however none of the cheapskates using GitHub as a CDN would ever consider paying for a CDN...
I can't blame them (being GitHub). Bandwidth and storage can be expensive.
That being said, I think it's inaccurate to characterize the people using GitHub as a CDN uniformly as "cheapskates". Many are just open source developers, often of limited means, trying to expose their work to the largest audience possible.
When you just start your project you may use github as your CDN and hope no one notices, but when you are as big as homebrew using someones elses resources like this is a pretty bad practice.
Github brings additional value that just being a DVCS - I manage all of my external consultants through it (assign work through issues, often review/approve PR's via the UI, able to easily share a link to a specific line of code, easily share info via the wiki etc).
There are obviously alternatives but the Github UI just makes it so easy to work with others.
The features that github has would be pretty hard to migrate even to something that has almost 1:1 feature parity like GitLab. I don't know of an export tool they have for all the data associated with github.
