Theory of Facebook->Reddit?

Why are threads from Facebook specifically reposted so much more often than threads from any other forum?

It's the most popular social network on the planet.

Why are FB threads so often hostile or nearly hostile? Aren't the people you're talking to supposed to be your friends?

Why would people post agreeable FB conversations? That's boring, and people are usually posting FB shit so they can feel superior to the person they were shitting on.

When you meet these people in real life, does the Facebook hostility translate into real-life tension? Or is it ignored/laughed off?

Depends on the people involved. Most people don't have stringent requirements to add people on FB. If you've met them once and can kinda remember their name, that's enough for a lot of people. Also, there's lots of people who have others that they are forced to be FB friends with, for better or for worse. Relatives, co-workers, any others who might get butt hurt if you reject their friend request. Also, things that are disagreeable, especially religious or political topics, aren't really fit conversation topics in polite company, but people can get away with it online because there's so few social cues to take into consideration.

Edit: typos

Why are threads from Facebook specifically reposted so much more often than threads from any other forum?

In part, I'd say it's a ritual of self-justification. Most of the Facebook screen caps I've seen on Reddit come attached with the (stated or unstated) message of, "Look how stupid and vulgar the rest of the world can be." Which implies that we're somehow better.

For r/atheism in particular, Facebook seems to represent the outside world, which is why you see a variant of FB cap in that reddit that you see virtually nowhere else on Reddit: the "should I post it" cap. That's when the author types a response into the "reply" field of a person's Facebook wall, but rather than actually send the response, they take a screencap to show r/atheism the clever or devastating response they would have sent if they didn't want to preserve some social status quo.

Why are FB threads so often hostile or nearly hostile?

I'm not sure that there's any more hostility on FB than you'd find on Reddit. But you're not likely to see a FB comment on Reddit unless it's egregious in one regard or another. Usually, that means the people involved look either terribly clever, terribly stupid, or just plain terrible.