I try to offer "gl hf" and "gg" as traditional sportsmanship messages, but I don't always get them in. Not that I mind if someone omits them; sometimes you're in a hurry or preoccupied, after all. Or you might just be a jerk; but I'll give you the benefit of the doubt.
The only thing that bothers me is when people complain that I didn't greet them or chat. I play a lot at work; I don't always have time for a conversation in every game. If they're just whining about how I beat them with a "jerk" strategy (e.g. King's Court megaturns), I mostly just dismiss it.
On a particularly good game, or one in which my opponent crushed me handily (and not just due to my mistakes), I'll offer a "gg wp" (good game, well played) instead. Other times I might use the chat to gripe about exceptionally bad luck (like
the world's worst Loan) Sometimes this leads to a friendly conversation about the game, which is very satisfying. I also was quite impressed recently with the manners of a player who apologized after breaking out and running away with a Tournament game after a mirrored opening. That's going above and beyond common courtesy, and I was glad to tell him I bore him no hard feelings (runaway leader is the face of Tournament games, after all).
tl;dr I don't think there's been a decline in civility; feels like there's about the same mix as there has been for a while. A little bit of everything.