I've never been good at saying goodbye. If it's someone that I'll never see again, all i can think of is "bye, have a good life" which probably isn't the best way to drive closure to an 'acquaintanceship'. So I end up not really saying anything at all.
If it's someone that there's a possibility that I'll see again, I don't see the point of a long drawn out goodbye because chances are, our paths will cross again some time in the future, specially considering how easy it is to stay in touch with FB/social media nowadays.
If it's someone that I will truly miss having regular contact with, I also don't say much to belabor the goodbye because I tend to tear up easily and I don't like to cry in public because my face becomes one big swollen mess.
So if you are someone that I have to say goodbye to, I apologize in advance for the brevity of my goodbye but know that it's not because I don't care, it's just because I'm bad at saying goodbye.