That's it - no explanation, no apology, nothing.
Of course this will leave you devastated! This is a terrible thing to do to you, and action must be taken immediately.
However, by taking action, I don't mean that you must get on the phone and call your ex and plead to be taken back.
On the contrary - give your ex just what he/she wants, and you'll see, this reaction of yours will make your ex regret breaking up with you so badly that he/she will be running after you before long! If you feel that you can't possibly talk to your ex without breaking down, then rather send an email.
Tell your ex that you're okay with the break up.
Say that if the relationship upset him/her so badly, that you totally agree with his/her decision.
As an afterthought, add to the email, that you have already been out a few times with your friends, and that there is this very cute someone who you are interested in, and, since the relationship is over, you might just decide to hook up with this person.
Yes, it will make your ex jealous, but it won't be for long, I can assure you.
Remember, you'll be with your friends, and EVERYTHING you say and do will get back to your ex in no time flat, and if you haven't done anything wrong, this will also get back to your ex.
To make your intentions seem real though, go all out to get yourself looking really stunning when you do go out with your friends.
This WILL get the members of the opposite sex interested in you, so you won't have to make any effort there.
Your indifference to the break up, the way you look, the interest from other guys/girls, and what your friends say about you are all going make you rex regret breaking up with you in a big way! Was he/she absolutely mad to even think of ending the relationship?