Are you not satisfied with the product or service you bought? Did you try customer service via phone and got tired of listening music?
You can just send one tweet or public facebook message and magically company will contact you in short time. Almost all major companies are providing customer service via social media so you should use it to get better customer service.
Are you happy customer? One tweet or public facebook message can help the company in promoting their product or service.
Also it can help your friends and family before purchasing same product or service.