Addicted to those cute little games like FarmVille and Mafia Wars on Facebook? You might incur unexpected charges on your credit card if you're not careful, according to a new class-action lawsuit.

Facebook and social gaming company Zynga are being sued for $5 million over something that's known as post-transaction marketing.

Let's say you're at Facebook connecting with your friends. While there, you accept an offer for a risk-free trial as part of an ad that might be embedded in a game. You're typically asked for a credit card or checking account number to participate in the supposedly free trial -- and then your account is dinged.

Facebook has so far denied any responsibility for this post-transaction marketing. Zynga, meanwhile, allegedly made up to $80 million from these offers.

But this is not just Facebook doing this kind of stuff. The Los Angeles Times reports RealtyTrac has been accused of working with a post-transaction marketer that charged people for $45/month for a supposed subscription.

The takeaway here for you is be sure to check your credit card statements and bank accounts each month for unknown charges.