When you click the link to Top Profile Viewers, you will be prompted to fill out a questionnaire and required to click through a link 'to verify your account'. Finally, your friends on Facebook will be spammed with invitations to click the link too.
The purpose of this scam is to (a) gather personal information about you, (b) gain a commission when you click through their link and (c) propagate the scam on to your friends.
If you have clicked the link, you should go through your feed and remove any references to the scam and also remove the event that has been posted to your wall.
For more information about this scam, please consult the links below.
http://www.facebookfakes.com/?p=341
http://www.facecrooks.com/safety-center/scam-watch/item/1257-scam-alert-my-top-profile-viewers
http://romcartridge.blogspot.com/2010/01/is-it-possible-to-know-who-views-your.html
The next time you think about downloading the next free widget, remember that if you're not paying for it you are not the customer. You're the product.