I think the current answer fails to mention two important things: 1. GitHub. 2. Linus himself wrote it (the kernel previously used BitKeeper but that never got any real traction AFAIK). Having said that, I think this phrase totally nails it: "If it's good enough for the Linux kernel, it's good enough for you" is a very convincing argument.
GitHub was founded in 2008. The spike is in 2010-01. That's what the question mentions too.
for eg. it is comparable to the chart of Facebook growth[0]
[0] http://www.nathaninfocus.com/wp-content/uploads/2011/01/Face...