1) you're missing a few figures and the whole picture, by the conservative estimation Zuckerberg will make 16 billions with the ipo so he personally made ~20 bucks per registered user. Selling ads is pocket money, at the end of the day the wealth is in the mega database of the user data.
See, paying users can choose to take their dollars elsewhere so you have to give them what they want, if it's gratis people flock on the service and lack the incentive to go away if the service is somewhat unsatisfying so you can impose whatever you want to them.
2) they can't offer such an option, facebook is a machine designed to collect, store, hold and aggregate user data and they built a business model out of it. Putting the user in control of its data is the opposite of what facebook wants.
You can't really compare the subscription of news aggregator websites and an hypothetical subscription on facebook. Facebook is unique in the fact that it holds user personal data and has a strong grasp on it through heavy vendor lock-in, you can easily leave slashdot or reddit and get your news elsewhere from an alternative but you can't leave facebook easily because they have your data, data you need in your daily use and there's nowhere to go to to find an equivalent.