(When they were the ones that dumped you)
Most Helpful Guy
Most Helpful Girl
There are a few possibilities to your question.
1. They want to make you sad or jealous or both
2. They want to prove that they are better off without you
3. They want you to know that their new patner is better than you.
Actually by doing that the ex girlfriend always wins because it means that the ex girlfriend is still important to the former patner.2