It's possible that I'm misunderstanding Sourceforge's goals, but for a site that is ostensibly about hosting software development, having the developer-oriented part of the experience be less convenient than that of an end-user seems backwards. I'm ashamed to say that I've never had anything on either Github or Sourceforge, but Github seems to be very much about sharing the sources, whereas Sourceforge seems to be much more about conveniently distributing the project's binaries.

This is probably largely related to the behavior of their users, though: Github's interface is not a lot more informative than Sourceforge's if no one writes a useful readme, and there have frequently been well-written projects hosted on Sourceforge that have lots of good information on how it works, or how to use it. However, I rarely seem to hear about new projects at Sourceforge, and frequently do see stuff at Github that has well-written introductory documentation accompanying the sources.

I frequently want to read about projects that solve interesting problems, but might not have the interest in installing or using it. Moreover, many people seem to now be using Github as a "Host my interesting document easily" host, which reinforces the "I go to Github to read code / read about code" perception. I'm sure a large part of it is confirmation bias, since most of the interesting code-related things I read about here are hosted at Github. ;)