I have had this experience a few times and it put me off of using Facebook completely. Occasionally I would mention something (not emailed it googled, just mentioned out loud) and I would get an add for the product described. I did figure if Facebook was literally listening to my mobile phone mic someone would have found out about it.
A friend suggested an experiment: say an obscure, but random word a few times in conversation and see if Facebook reacts. I’m tried; it didn’t.
A recent article from USA Today describes how Facebook does this. While it doesnt actually eavesdrop on you, it does something maybe even creepier.