It's been a few years since I've been a regular (Free)BSD user, but I think github has a very different use case.
The ports tree is for handling distribution of compilable/installable software for the platform, and you synch it from a server, then build. It's not really intended to promote the branching and modification, and social aspects that github does.
I largely agree with your points about branching but less so about modification. It's easy to submit patches though they do go through a sometimes rigorous code review. Does github have a formal mechanism for code reviews? Does that differ from Sourceforge, Google Code or other source repositories <http://en.wikipedia.org/wiki/Comparison_of_open_source_softw...?
I don't mean my mom, but I do mean to imply (without evidence) ordinary non-Norvig coders who aren't steeped in Unix lore.
Does ports really do what Github does, or does it do what apt/rpm/yum/pacman/etc do?