The side that says NO has a simple reason: People should be working in office. Not socializing. Not playing. Facebook is playful, even immoral. Let us call this group High Priests of Scientific Management. These gurus are usually the Command-and-Control guys who believe that office work is about putting a bunch of kids in a cage and giving them some simple menial tasks to perform.
This is where they get it wrong. They forget that office work is no longer like that. May be, they don't get because they are mostly managers: They don't work anymore. They sit in their cabins snooping around on other people, forgetting that it is social connections and free energies of the educated adults that determine the fate of businesses today.
The 'bosses' dont get the fact that slavery is over. Finished. It is illegal, I know, but it is unproductive. That was exactly the reason why slavery was abandoned as a practice, not for any moral reason. A free man (or woman) is more productive - physically and intellectually. The trick that managers must perform in today's workplace is to make employees feel at home, and make them feel that they are running their own businesses.
Indeed, some managers follow what I am saying: They demand that the employees must feel at home at the office, consider it to be their own business. But, no facebook, please; no personal work! It is indeed a strange home that is built like prison.
Consider the other part of the argument. Business is intensely social. It is about knowing people, being transparent and connecting. Today's marketplace is FACEBOOK. Today's reputations get built and destroyed in Twitter. If you ban Facebook in your office, you are doing what hermits used to do in the ancient ages: Leave the city. Good luck to you.
Company IT guys mostly hate Facebook. Strangely. But, then, to be fair on them, that's survival instinct. They don't get it. The reason why they are company IT, and not IT IT, is because they are already clutching the last straw on the edges of the sink of obsolesce. They must fight on for a few minutes longer. People can't just open their computers and connect to the outside world without first asking for a gate pass. Note that most these guys are Gen X, and they find the Facebook lot almost impossible to put up with. They resent the Gen Y because they are not part of it.
The problem is, though, you can't remain neutral in a battle like this. You have to take sides, you have to let your views known. Because this is a more fundamental battle about how you see your company. Are you building a prison or a business? Are you talking about bringing people in or driving them out? Do you want smart people or dumb people to work for you? Do you want to know what the outside world is talking about you, or put your head in the sands? Do you want to connect to Generation Y, today's consumers, or are you going into retirement?
In the end, one more thing to the people who wants to have it both ways: It does not work. So, if you are a naysayer, my advice: Get real. Let Facebook come to work for you.