"an account that a user maintains in addition to his or her principal account."
The problem there is that a person trying hard to keep personas separate can do so without Facebook being able to track (I assume they have to be more specific than just matching by IP address).
Re: Matching IP address - given the prevalence of NAT, in which several thousand users will be associated with a single IP address - you can't rely on that.
Cookies, on the other hand, or, browser-fingerprint-matching, probably can be used to identify these shill accounts. I wonder if Facebook knows how many of it's accounts are fake, or if they actively avoid looking into that information.
Then again, while it is common for us 'computer folk' to have more than 1 computer, a lot of households have just 1 shared by many people. My sister's family has 1 shared computer with 3 Facebook users (soon to be 4 in a year or two).
The problem there is that a person trying hard to keep personas separate can do so without Facebook being able to track (I assume they have to be more specific than just matching by IP address).