While the writing of an angry email or post is often incredibly cathartic, posting it is usually about as effective as writing a really angry letter and sending it. You might feel better or vindicated now, but you’ll probably feel a little silly later – and you’ve the potential to do a ton of harm to the person(s) addressed.
Sometimes it’s just better to rant at a friend. Or, to write it and appreciate the catharsis, and then sit on it for a day or two and decide what you really want to say. And then try say it in a way that is clear and expressive without being combative/vindictive. While it doesn’t have the self-satisfied, smug sensation of really firing off a zinger, it’s more likely to produce positive change instead of getting you blown off (or worse).