Fall off a cliff
Definition 1 of 3
Pro Tip 1/3Business Context
Use this phrase to describe a drastic drop in business measures, like sales or profits. 
Our profits fell off a cliff once the new competition entered the market.
Pro Tip 2/3Dramatic Impact
It implies a sudden, severe impact, often surprising or unexpected. 
Viewer interest fell off a cliff once the TV show killed off the main character.
Pro Tip 3/3Negative Connotation
It has a negative connotation, suggesting something went very wrong. 
His approval ratings fell off a cliff after the mishandling of the crisis.