They think that sharing a letter will automatically make a positive effect. It can have a negative effect when people comment on it and show the letter writer that, perhaps, they made a mistake.

Shared letters are effective when the expose, to the public, a problem with a company. When people reading the letter join in and say "you're right... the company was wrong... thanks for telling me... I'm never shopping there again..." it has a major impact.

Problem is, in a letter like this, others don't agree with you.

Have you considered that the reason people are arguing with you is because, maybe, you might be wrong? Or maybe it's not that you're wrong, but you wrote your letter in such a way that it didn't convey the message?

This is a site about feedback. Given the responses you've received, consider this as feedback. Consider what you might have done differently based on what people are telling you.

In the end, if you feel that you're right and everyone else is wrong, that's your right. But doesn't that mean that you fundamentally believe that feedback isn't worthwhile. And doesn't that mean that it's pointless to even write letters like the one you wrote?

"According to Planetfeedback, shared letters are more effective."

Just because they've said it doesn't make it true. At least, not in all cases.