One of the things that I am guilty of, is neglecting to wash my compact powder sponges. I wash my brushes every fortnight. Because I don't see my sponges, I don't wash them as often. (OUt of sight, out of mind.)
It is very important to keep powder sponges clean because it comes into contact with your face often. When you touch up, your sponges soak up your sebum from your face and that gets transferred onto your face powders. That is why sometimes when you look closely at your compacts, you notice some oxidised portions that have become really hard.
It is very simple to wash sponges. I squirt some body soap and massage the soap into the sponge. Then I rinse it well with water and leave to dry. It has to be completely dry before you keep it. Bacteria grows well in moist things, you don't want your sponge to be a hot bed for bacteria.
My sponges usually do not turn out as pristine as before but I feel better using them on my face. How often do you wash your sponges?