The correlation is backward. RSS usage had flatlined as Twitter/Facebook took over the use cases of RSS for most people; with RSS in decline, Reader was killed.

The correlation is backward. RSS usage had flatlined as Twitter/Facebook took over the use cases of RSS for most people; with RSS in decline, Reader was killed... and now, years later, so is this Feed API. Yes, you like RSS. So do I. But most people never used it. Feedly has 15 million users, and they estimated that they captured 85% of Google Reader's user base when it shut down. Twitter is 20x larger. Facebook is 100x larger.