The guy/girl that is dated or messed around with until the user can get the guy/girl that ditched them to come back.
Often the user was just a place-holder for said ex and didn't know it/can't get over him/her.
girl:Oh yeah I can't wait to hang out with you tomorrow <33!! love you baby
guy:*awesome this is love ^.^*
The following Evening:
girl:Yeah...I'm back with <insert douchebag that just ruined your fucking life> and I wasn't really in love with you.
guy:*Holy fucking shit I'm going to kill myself as soon as I finish thinking this.*
Perfect example of a Place-Holder...often suicide IS the result of being a Place-holder.
Other results are binge-drinking and death threats.