When an ad pops up in your news feed that is irrelevant to you, it's ANNOYING! So, when a promoted post is not optimized to reach the perfect audience, you can bet on receiving some negative comments.
When promoted posts are experiencing negative activity in the form of comments, hides, follow drops, etc., this is an indication that you are spending money to reach the wrong audience. And remember that on average, for each consumer that complains, 26 chose not to express their negative opinion.
Targeting is certainly not a new concept, you covered that in marketing 101. So don't allow that quick and easy "Boost Post" button to make you forget the basics. Take the time to dig deeper into the demographics, interests and behavior options available to you, then target, test and measure.
With the depth of user data available to us from today's social media channels, there's simply no excuse for wasting social media advertising dollars to reach a demographic that is not a perfect fit for your business. Revise your target audience, messaging and graphics until your ad is performing optimally and you'll not only improve ROI, but the headache of negative comments will likely disappear as well.
And the next time your post gets hit with an obnoxious negative comment, you can tell that troll, "Thank you for helping me improve my ROI!"