I'm friends with the majority of my exes, but not all. I think I've been lucky that most of my relationships have been mutually respectful and supportive and the breakups have kept that tone.
There are a few where friendship wasn't possible.
I don't hang out regularly with any of my exes, but we keep tabs on each other. Occasional texts or emails or calls, a few facebook friends. Just check-ins to make sure each other are okay.