What constitutes “commercial purposes”

I'm looking at the Creative Commons Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0) license.

It says that I can use it for "non-commercial purposes".

What does that mean? I see in Stack Exchange, under Network Profile there is that graph that tracks your points across your Stack Exchange accounts. It uses a control called HighCharts which have a paid and Creative Commons licensed version.

So would Stack Overflow constitute a commercial site? We don't pay to use this site, but obviously the site makes money from ads, etc.

Then again, there's a lot of sites that have ads who won't necessarily make a profit, it may only be subsiding their costs. But even then, you could argue that even if it is only subsiding their costs, a lot of IT companies run at a loss in order to build a big enough customer base.

So where is the line here? Is it any website on the internet? Is it any website that has ads? Is it any website that turns over a profit?