What's the etiquette?

While I've argued before that sites like Facebook and MySpace are making dating more difficult (such as when one person changed their status to "in a relationship" before they've had "the conversation"), it is also starting to affect friends of people in said relationship.

For example, a long-time friend was dating this girl for a couple of years, so naturally, we became Facebook Friends (which is like a whole new level of friendship that says "I accept that we get along even though we'll never hang out alone"). Now that they've broken up, should I de-friend her?

And what about ex's -- do you defriend them too? What about if you become actual friends again down the road, do you re-add?

And what about if your friend hasn't de-friended his ex? Do you? What if he has?

These are the questions that keep me up at night.