At The Guardian, Danuta Kean wrote a very good post about people who feel compelled to tag writers on social media with their very negative book reviews:
Others seem genuinely to think they are helping the writer by telling them they hated their latest work. All seem blissfully unaware that as a far as the author is concerned, their feedback is unwanted. Their tweet or post is on a level with a stranger shouting abuse in the street.
Remember: authors are humans, too. Maybe don't make them feel like shit unless it's absolutely necessary? This isn't to say that you should skip writing negative reviews. You just don't need to wrap your negative review up in a bow and then drop it in an author's lap.