I understand that Facebook is faceless, and even if you know the person whose post you're commenting on, you are not speaking into someone's face. You are not looking into their eyes, and you're not in their physical presence in order to read their body language and understand their reaction to your words.
However, I'm guessing that people who are aggressive communicators on Facebook are probably aggressive in person as well. So let's talk about the many ways you can stifle conversation instead of cultivate it, using examples from Facebook comments and comments I've read in online news articles.
1. Ask a question that you don't want answered. Read my post about rhetorical questions here.
"What ever happened to journalistic integrity - the who, what, when, where, and why of the facts and theories without conjecture and assertions?"
2. Attack with language
This includes name-calling, labeling, attacking the person instead of disagreeing with their ideas, saying things like, "Why don't you...?" "You should just..."
"Actually NO that's not true at all and merely the idiocy of your bigoted addled brain."
3. Show impatience with your audience when they take [what you perceive to be] too long to work out solutions or respond to your examples and questions.
"Come on, people! Nobody can tell me the answer to this question?"
4. Patronize or condescend to them
Use words like "honey," and "sweetheart" or give the impression that you think the person is beneath you, even if said in a "friendly" way.
5. Use sarcasm carelessly or direct it at individuals
(Imagine the eye roll...)"You mean Eurosocialism leads to brain-dead people with no initiative, no ideas of their own, no responsibility, or ability to manage their finances? You mean it isn't the job of government to wipe everyone's bottom, cradle to grave?"
6. Judge others based on perceived negative assumptions
"And the inner totalitarian of the modern 'liberal' slips out."
"I bet you're right wing -- let's go see."
7. Insist on being right or refuse to acknowledge the validity of another person's point of view
Refuse to concede an argument or even agree to disagree. Every disagreement is a win/lose proposition.
8. Judge people for not being like you.
"Comments like yours are exactly what is wrong with this country and where did you get your statistics from, Wikipedia?"
It's unlikely that a speaker will yell or get visibly angry with an audience member, but these other communication issues listed are clues to aggression -- and they're just the verbal examples. I didn't even go into nonverbal aggressive communication, so feel free to share your examples in the comments!
Bottom line: Your audience members may feel angry, embarrassed, humiliated, hurt, or frustrated, but whatever they feel, they will not feel engaged with you.
Be honest: Are you communicating in any of the ways listed here? Read your Facebook status updates and comments, your tweets, your e-mails and listen to your own utterances. And ask yourself: Am I intimidating people into silence? Am I acting like a bully? Do I always have to be right?
Am I inviting conversation or shutting it down?
Want to get your message across, but in a respectful, thoughtful way that encourages dialog? Read this post on 8 tips for assertive communication.