assuming you were in an exclusive/monogamous relationship.
I never have, but I kinda wished that I had with my ex.
there was a period of about 6 weeks where I knew that I wanted to breakup with him, but hadn't figured out the best way to do it yet... during that period, I met another guy at a party. he was cute and funny and kind of flirting with me. I really wanted to ask for his number (or slip him mine), but it just felt to skeezy to do it while I was technically still in a relationship.
after the breakup, I tried tracking him down but to no avail. there's only one person that I knew of who could put me in touch with him, but she was really good friends with my ex and had all her privacy settings tuned to the max on Facebook, so I couldn't even browse through her friends looking for a familiar face.