The vast majority of the traffic that hits Sourceforge is there to download software, not contribute to the projects. When we spoke to them a few years ago, they said their main competitor was Downloads.com not us (GitHub).
Don't get too caught up using graphs to assume relevancy, Myspace still gets a lot of traffic.
We use both. SourceForge brings to the table a huge distribution network. Webmin burns terabytes of transfer for millions of downloads every year. I'm not sure github is even in the game on that front, are they? They also host the webmin.com domain, for the time being, which saves us some bandwidth, as well.
But, we never used the VCS at SourceForge, and we barely touch the forums and bug tracker...if we could turn them off, we would.
My feelings exactly. SF still looks like some cheap JIRA install, not a hacker-friendly user-facing site like GitHub.
I get the impression they're undermanned, underfunded and under the weight of their technical debt. You just can't do all the cool things GitHub can when you're supporting SVN (do they still do CVS too?).
They support cvs, svn, git, hg, and bzr. Very impressive.
Overall their website navigation seems to be heaps better. I know this rewrite has been a long time coming. I know it's also been done in python, and by some quite good developers - so I'm guessing they will progress faster now (assuming they all still have jobs).
On this front, I think SourceForge deserves a lot of respect. They've weathered acquisitions, had to find monetization methods, and through it all they've remained dedicated supporters of Open Source. The amount of money and effort they've invested in Open Source development infrastructure is admirable.
It's not beautiful, and it's got a lot of warts...but SF.net has its heart in the right place, which is something github has not yet proven.
I'm not sure there are enough network effects to make it a winner take all market. Especially since systems like git make it so very easy to migrate elsewhere.