Who is the "you" in "as you would expect"? Would you also expect Enter to insert a line feed in an IM client? How about Twitter? It's all a question of mental model.
It makes sense to insert a line feed in a system built for verbose comments, which probably also support rich text or at least some basic formatting. 99.9% of comments on Facebook are one-liners at best, with a huge mass of comments being 3 words or less (at least based on my own observations, don't have any data to back this up). I don't know you of course, but I bet that even if you observe yourself for a couple days and count the number of times you leave a comment that needs a line break, you'd see that it hardly ever happens.
Add to this the already very high and rising mobile usage of Facebook, where people are even less inclined to type.
In a system like that it makes no sense to have Enter insert a line break, you'd basically frustrate the absolute majority of your users for most of the time.
And just to be on the safe side - I assure you that the comments aren't short because Enter submits, but rather Enter submits because the comments are short :).
Facebook actually gives you an endless grace period when you edit a comments, since you can always edit it. If you want to see a short grace period, look at StackExchange, where you can only edit a comment for a second or two after you post it, and then you can only delete it, but not edit.
Regarding the emails being sent immediately - what's a better way? How much would you have them wait? A minute? An hour? What prevents someone from wishing to take their words back after a day or a month? There's no way to arrive at any sort of an "effective" delay because the use case is so rare and chaotic, and in 99% of the cases it would just be perceived as a bug. And why not ask for the same grace period when you send a regular email? People often regret emails they sent. Gmail had actually experimented with this a while back, but I don't think it made it into the main functionality. As a designer, you can either send the email immediately or not send it at all, anything in between would be a worse design solution.
I do think that Facebook tries to manipulate their users, to make the most out of their business model. But they do it mostly through careful manipulation of the feed, not through the mechanisms you described. And in general it's a great idea for people to think before they speak or comment, so they wouldn't have to take their words back :).
