alt Hacker NewsMy money is on the likelihood that most GMail users started marking these emails as spam, and GMail recognized that overriding trend and began to redirect the emails accordingly on a broader scale.
Essentially, the people FontAwesome thinks will want to hear about their new features have actually, collectively, said, "No thanks," and FontAwesome is struggling to accept that.
I think part of the problem is that Google has conflated the "mark as spam" button with "unsubscribe" and people just mash it as a shortcut to "make this email go away".