CNN viewership dropped about 40% in 2018. I am not sure it was a result of boycott (promoted by Fox) or simply bad journalism.
I was asked to make references. Here are two:
Forbes states a 41% drop of the most important viewers
More about total ratings of CNN vs Fox and NBC.
The campaign against Breitbart organised by among others the Twitter account Sleeping Giants may qualify. Breitbart lost approximately 90% of their advertisers over a three month period in 2017 and have not yet recovered. They have also since then lost about 75% of their readership. The boycott was explicitly motivated by accusations of spreading fake news ...