Don't you think it's more likely that they just aren't noticing the ad before it is relevant? I mean, how often do you really notice an ad anyway?

That being said, perhaps the ads are doing their job and planting the idea for that particular product. You then bring it up in conversation or mention it out loud. Then, when you return to facebook later, you see the same ad again and due to it's recent mention, it jumps out at you.

Both seem more plausible.