I know your ego is bruised from being dumped but look it at from this perspective.....she did you a favor. Now you're free to enjoy the single life again without having the guilt of breaking a girls heart.
Go drinking with your buddies, take up a new hobby, go hit on some girls. You're a free man now.
And don't read too much into her facebook message. You don't need to be analyzing this sh*t with one girl when there are so many other options out there. Remember the abundance mentality. Ideally, I wouldnt respond just b/c that chapter of your life with her is over so there's no reason to dig up old garbage. But if you decide to anyways, just keep it along the lines of something like "im doing great, hope things are going well for you also." Short and civil.
Basically, unless you just want to keep this girl around as a FB, you need to move on and find new girls.