Your ex-gf/bf messages "Happy Valentines Day"

Most of the short responses come on the passive-aggressive, ofeensive side, which is great if you REALLY want to show them that whatever they did really got you down and you're still hurting. If it was that bad, you probably want to show them you're doing well and don't particularly care about it.
The "proper" response would be cordial, but not intimate or warm of course. This may be hard to pull off sometimes, but besides being the proper, right thing to do you never know when you run into them with them (Or their friends) in position to do/decide something over you.
The "shorter than their message" may have some truth, if you want to send the message that you've moved on and don't want to give the impression you're open for more. So I actually like the "Have a good one too" as it simply just returns it to them, I was going to suggest "Thanks, you as well" but it just sounded dumb so I went with the full spelled out. That may not be good if you aren't open for continuation.
And yeah, writing too long & late now, haha ;)