While having more than one Facebook account is technically a violation of Facebook's terms of service, countless people do indeed start secondary accounts for various reasons. Some do it for privacy; so they can post semi-public information that's still hidden from their wider social network. Others start pages for fake characters simply as a joke.
But many others start up their secondary Facebook accounts for gaming purposes. These multiple-account users can divided into two main groups. The first group creates multiple accounts to gain an edge in a game -- after all, it's a lot easier to get dozens of neighbors in Farmville if all those neighbors are actually you.
The second group simply starts up a secondary gaming Facebook account for convenience. Not only does this secondary account stop all those "free gift request" messages from clogging up the News Feeds of your real friends' News Feeds (who probably couldn't care less, if they don't play those games), but it also protects your "main" Facebook account from in-game neighbors who may be complete strangers in real life.
It's hard to determine exactly how many people have used multiple Facebook accounts for either of these gaming purposes, but the Facebook chatter about the problem is not insubstantial. Facebook's own official statistics show that only 50% of their users log in each day, though it's hard to extrapolate what that means to the "real" user count.
Still, as Facebook continues to grow as a gaming platform, it seems they're going to have to take some action on the multiple account problem. Either they'll have to get much more proactive about policing their one-person-one-account policy or loosen the regulations so users can easily separate out each facet of their online persona (such as their gaming persona) into its own unique account. Then, at least, Facebook can honestly say that their hundreds of millions of accounts each represent a unique personality, if not a unique person.