And now for the best tips on handling an argument online (especially on Facebook, where these are people you love and feel affection for, argument notwithstanding):
1. As much as possible, never make it personal.
a. Do not point out flaws belonging to the person with whom you are engaged in an argument. e.g. "PANGET!"
b. Do not try to ruffle feathers that shouldn't be ruffled. Stick to the merits raised in the debate.
2. Avoid referring to yourself too much.
Passive sentence construction and third person perspectives are important in convincing everyone that you are not hung up on yourself, but focused on the concept you defend.
3. Know your shit.
a. Know what you believe in and be sure you've thought it through.
b. Do not be afraid of being called tasteless. Just make sure you are well-informed.
Your responsibility is Truth and clear outlining of the semantics and labels of all terms used in the discourse. When the truth is ugly, then pointing it out usually garners much criticism and judgment.
Having said all this, be prepared for the variety of opinions when posting anything controversial. People you love do not always see things the same way you do. This does not make them monsters, nor does it make you one.
Promise yourself open-mindedness and you'll be all right. Do not mistake this for upholding principles you believe in. The former merely means that once you arrive at the latter, then you know it is something worth arguing to defend, and heck even die for.
Good luck.
"Science cannot resolve moral conflicts, but it can help to more accurately frame the debates about those conflicts."
- Heinz R. Pagels