Tumblr peaked (in google searches) in mid-2013: https://trends.google.com/trends/explore?date=all&geo=US&q=t.... Peak Alexa rank was at #23 in August 2013 according to this: http://tumblr.com.alexarankdb.com/

FOSTA became law in April 2018.

So please explain to us how FOSTA was responsible for Tumblr's demise.