I can't see Github going the way of SourceForge. SF was exclusively a download portal for FOSS, it needed ads (and, eventually, malware) to survive because it had no other revenue stream. Github, on the other hand, really only hosts FOSS to further cement its position as the standard in git hosting. Their bread and butter is large organizations who are paying per seat to keep their code hosted there.
There was an effort at VA Linux to convert SourceForge into an enterprise project. That failed. I believe that's when SF was spun off (along with Slashdot) as an independent. Or was it that SF became what was pretty much all that was left of VA after the hardware business left?
I'm a bit fuzzy on the history, and wasn't there at the time, though I do know people who were.
Point being that SourceForge was not intended to be exclusively FOSS.
