The bad guys, or hackers as we call them technically, use both internal and external tactics to gain access to your computer. The internal method is easy. Create a luring application and insert a malicious code or merge a malware code into a video embedding code and then post it on Facebook. When you click on the link to play the video or to open the application, the code downloads malware to your computer. As there is no restriction on creating applications, anyone can create such applications/games for use with Facebook. Though Facebook might remove suspicious applications, ads, and posts after receiving complaints, it may be too late.
The other method is to send emails to Facebook subscribers. Most of the people post their email addresses and smartphone numbers on their Facebook profile. This makes them easy targets for Facebook virus and worms. Read Dealing with Facebook Security Problem for basic information on keeping your Facebook account safe.
The most basic way to avoid a virus or any other malware is to avoid any post, video, page or application that looks out of this world. You can still have fun on Facebook, but it is wise to read user reviews of Facebook games before trying them. An example of recently video spread virus was titled, “Sexiest Video Ever." This is not available anymore on Facebook, but you need to be careful should anyone send the link via email. The next sections will talk about different known Facebook viruses and methods to deal with them.
Note: The founder of Facebook, Mark Zuckerberg says that there is no option of screening new applications before they go live as Facebook is meant to be an open space. This increases the risk of Facebook virus and other malware.