It depends on the nature of the breakup; whether it is a bitter split or a mutual split.
Obviously it will be easier to maintain a friendship if it is a mutual break up.
However there is no point being bitter over a break up, no matter how it happened.
You will only be hurting yourself by harboring hatred and anger towards your exes. All your hatred and anger will have no effect on them if they have already embarked on another relationship. You will just be piling all the misery on yourself.
Sure, it hurts like hell when the break up has just occurred but over time, if your heart is big enough you will learn to forgive and forget.
For me, I would like to wish all my exes well no matter how we broke up as I believe karma will catch up with them one day, be it good or bad depending on what they have done. No need for me to live in anger because of them.

I am married now and I would still like to keep in touch with my exes but the missus won't allow it...so that's that.