I know a lot of Second Life residents had a Facebook account for their avatar as an extension to their Second Life presence. Linden Lab even semi-encouraged it inasmuch as you can link your Second Life profile to a Facebook account.
Apparently Facebook had a massive cull on this last year and removed thousands of such accounts.
I also have friends who have an 'online persona' type Facebook account that they are less picky about who friends along with a 'proper' account that only actual friends are invited to.
So, as others have said, alt accounts are more prevalent than Facebook think.