The wrong part of that sentence is the word essentially.
GitHub is essentially a social/collaboration app for people who spin code.
The fact that it uses the git distributed version control system is an implementation detail.
The essence of GitHub is in the network of people who use it and how easy it is to do so. Not in the fact that its a (really really) good front-end to a particular tool.
I'll admit that I'm unenthusiastic about social media in general, so this aspect of GitHub is the one I use the least. I've always found their slogan of "Social Coding" to be a bit of an oxymoron as programming for me is primarily a solitary activity.
Generally I find the design of GitHub to be more refined than Bitbucket (e.g., the README's look better) but it's not something I'd pay for.
I'd argue that the real action for "social collaboration" on open source projects is where it's always been: on mailing lists. The project I've followed most actively the past couple of years has been Go, which is hosted on Google Code, but I very rarely need to go there since I'm on the mailing list.