Possibly coincidental, but July 1, 2013 was the date Google Reader was discontinued.
You could also ask what happend in Nov 2011 that stopped the exponential increase
That was the month they got rid of sharing and encouraged Reader users to share through Google+. It also inspired this tweet, which I remember fondly.
https://twitter.com/jkottke/status/131434505947979776
Looking like @newsblur is the @pinboard of Google's deliciousing of Reader.
1 Nov 2011, @jkottke
Re:Ffffound is shutting down
by nobodynoes on Wednesday April 26, 2017 @02:36PM
(#14205709)
Pinterest probably killed it.
Re:Ffffound is shutting down
by vicentereig on Wednesday April 26, 2017 @02:38PM
(#14205733)
Unsplash.com seems to be gaining some momentum in comparison since Jan 2016 https://trends.google.com/trends/explore?date=all&q=FFFFOUND...
the decline in 2015 might be the push by google for mobile friendly sites.