Facebook is trying to make their user experience more secure on their best social networking website by providing free licenses for 5 main Antivirus Engines free for 6 months. These license keys are valid for 6 months and will work like a charm as all these company's has tied up with Facebook and wants to make every computer safe and sound.
So Facebook Antivirus Marketplace is the place which provides all those Antiviruses for free. It includes Microsoft Security Essential which is free, Norton Antivirus provided free for 6 months, McAfee Antivirus provided free for 6 months, Trend Micro Internet Security provided free for 6 months and Sophos Antivirus for Mac provided for free.
Well but you are only allowed to download 1 Antivirus of your choice by clicking Download Now option on this Antivirus Market place Page, click on download now and than click on Log in using your account so make sure to give it access and than like the Antivirus Page and than your download will start.