User X visited your site and bounced. Now you need to show an ad to convince him to come back.
If this is a solution to visitors bouncing, this seems to be going about it incorrectly. You should give them a good reason to stay on your site in the first place. If this is a solution to visitor retention - as in people leaving because they used your site and forgot about it - then you need to provide them a reminder that you exist as well as some ongoing value proposition.
I guess if this is delivering ROI though then I'm totally wrong.