If you are seeing an icon in the upper right of your screen that looks something like this,
then you've been selected by one of Facebook's partners to participate in an offer from them.
These are usually referred to as an Offerwall, and they are managed by a company called TrialPay. This will reward you with in-game gold after you make a purchase or perhaps sign up for a trial with the partner you were shown an ad for. Gold will usually not be deposited to your account until verification of the offer terms being completed.
If you believe you've completed the terms of the offer and you haven't yet received your gold, you'll need to return to the Offerwall and click the support link at the bottom- we have no control over who is offered these, and we cannot confirm whether or not the offer has been completed.