While I can't disagree that sourceforge failed, it was definitely more than a mere code-host.

Sourceforge had integrated mailing-list support, bug trackers, page-hosting, and more.

There was a community there, it was just that each community was based around a particular project. There was little chance of a user of project A from interesting with project B. But I guess the same could be said of github.

Sourceforge failed in part because of feature-creep, and availability issues. But I think it would be unfair to say that it wasn't "social".