Full Study: http://edgerankchecker.com/blog/2013/12/the-unofficial-official-study-of-reach-since-december-2nd/
September’s Reach was approximately 12.6%. In the week before the change, we were seeing Reach of about 9.5%. After the change, we’re seeing Reach of about 7.7% (a -19% change). So in the short term, we’re seeing a -19% change but from September to December, we saw a loss of -39%. The graph below shows that Reach fell from ~9.5% to about ~7.7%. Surprisingly, we did not see a drastic day in which a change was implemented—although many Admins reported seeing changes around 12/3.