Can you be friends with an ex?

After "The Architect" and I stopped seeing each other (please see my post Progress report: Five months of online dating), I was debating whether I should still keep in touch and just be friends.  I think he is a good guy and I don't harbor any negative feelings against him.  We were just not compatible.

There are situations in which couples decide to stay friends, or it is easier for them to be friends:

-  You have to.  For example, you share a social circle or work together.
-  You were good friends before and/or during the romantic relationship, and had a mutual or civil breakup without hard feelings.
-  You are both truly over the other person.
-  You are both dating someone else, and are both happy with your new boy/girlfriend.
-  You both may want to get back together later.
-  You are lonely and/or do not have a good support system (friends/family).
-  You want to help the other person cope post-breakup.
* Note that most of these situations require both parties to feel the same way.

But there are cons about being friends with an ex:

-  It is harder to get over them, and will hurt more if they have gotten over you.
-  Being with them brings back (good) memories.  And you don't think about the problems that led to the breakup.
-  You get mixed signals (and have false hope) because they are being friendly.
-  You do similar things as friends as you did when dating.  This may lead to confusion about your status.
-  There will be a romantic connection.  If you had your heart broken, you crave to get back together.  You want to kiss them but can't.  If you were the heartbreaker, you may feel uncomfortable knowing that your ex still has feelings for you.  You have to avoid situations that may lead to something romantic and avoid doing things that give them false hope.
-  They will eventually start dating someone else.  How will you act and feel then?  Can you still be their friend?  And if they end up getting married, would you attend their wedding (would they even invite you)?
-  You may not be able to fully disclose your personal life (such as who you are dating).  
-  You may not be able to always be emotionally supportive (such as being happy for them when they are dating someone awesome).
-  You may not always want to help them (e.g., help select an engagement ring for their fiance-to-be).
-  The new person you/your ex is dating will likely not be thrilled about your friendship.

I am not saying it is impossible to be friends with an ex.  It certainly is possible, as some of my friends have done it.  However, it is difficult.  The quality of the friendship may not be as good as a platonic one.  And if your ex hurt you or did not make you happy while you were dating, why spend time and energy on your ex rather than family and friends who truly care about you?  If you were the one who hurt your ex, be careful about giving them false hope if you stay friends.

Rachel and Ross on Friends: A great example of how difficult it is to stay platonic after a breakup