Delete it quickly, do not click on any links in the email
here is one example of one of the Facebook threats:
""The latest version of the Koobface virus arrives as an invitation from a friend or contact, when you click on that invite link you will then view a video from a counterfeit YouTube website. You are then told that you need to install an Adobe Flash Plug-in to view the video.
That is when a Trojan horse program gives you the Koobface virus. The worm takes control of the user’s computer, as well as their social networking account. To make sure that the ‘Koobface’ virus does not trick you, only click on links in an email if you are certain that the message is from your friend""
If you do a google search on 'Facebook email invite virus' you will see how the different virus/Trojans work.