> I’ve never stumbled upon a project by browsing GitHub casually.

Same.

The “discoverability” of github is two things:

1) Recognition of the name; so if I cannot find the source code and I believe something to be open source I will google the product name with “github”.

2) I can reasonably expect everyone to have an account, so getting issues and PRs is less effort than with other forges.

But I’ve practically never discovered some new FOSS from githubs social features, and I actually look at my friends activity feeds.