My guidelines on Facebook are to treat it like you would real life. And remember that posts on someone’s wall most likely go to all their friends and all of yours, so it’s not remotely a private conversation.
I would not get into an actual theological debate. I would argue moral things at times (usually when they bring it up), but then I would argue them within the framework of the person I am talking to. I would not at any point try to convince them that their religion is wrong, but I might occasionally try to convince people that Christianity is incompatible with something hateful they said. And, even then, I talk about it like I would in person, with all the softening I can muster.
People do not go on Facebook to be berated or have complicated conversations. It’s not necessarily that they want to just share something without replies. But they don’t want to get into a big fight. Just like you probably don’t get together with your friends for that purpose.
Now, an exception is when Facebook is just the comments section of a site. Then it works more like any other comments section, save with the caveat that someone who knows your real name might see it. Another exception would be groups specifically formed for the sort of conversations you are talking about.
And, of course, just like in real life, sometimes you wouldn’t have brought up the topic yourself, but are not averse to talking about it if someone else brings it up. Still, I think it’s probably good to think of it like a real life conversation, and not just an online debate.
Like it or not, the standards for discussion online and offline are different—at least, for the vast majority of people. Facebook is the weird middle child, due to being designed as a place online where you can meet with all your offline friends at once. And I think it makes more sense to treat it as offline talk.
It’s not even just the topics themselves. I find I’m just more personable in that environment, and less academic, for lack of a better word. I’m more expressive of most emotions, save for anger where I’m much more reserved.