There's so much wrong with this logic that I don't even know where to begin.
No doubt that, if the government wanted to track us, Facebook would indeed be a good way to do it. While a large percentage of the U.S. population isn't on the network, the amount of data analyst could garner from data mining Facebook could be enormous. The problem is, using Facebook as a source of information is a bit of a waste of time.
First, you have to deal with sorting U.S. users from non-U.S. users. While this would be a fairly trivial task - especially if Facebook gave the government access to its back-end database, it still is a hurdle to design around.
Next, and this is an even bigger problem, there's really no accurate way to verify the information someone posts to their profile. Sure, you could use sophisticated cross referencing and statistical analysis based on known data about the person (as published on their profile), use profiling software to generate a likelihood of truth factor, and all sorts of other magic tricks but, at the end of it all, there's no way to move further than knowing any information on a profile is more than 'probably true'. There's no reliable way to vest a users information.
Add to that the fact that, if you're the government, there are many other more reliable tools at your disposal through which you can learn more about a person than Facebook. Everyone carries a cell phone these days. Bug it, track it, whatever. This will probably give you MUCH more accurate information about the person than analysis their social profile will. People are less guarded on the phone than on Facebook and you're much more likely to get unfiltered, unguarded, reliable information.
I think this brings us to an even wider issue: when social networking isn't appropriate for you. The entire idea of social networking is to connect with existing friends and expand your social network (read: meet new people - people you don' t already know). If your reasons for joining a social network is only to connect with a closed group of friends and share information, then there are probably better ways to do it than Facebook. You might consider a private listserv or a Yahoo group, or even a privately installed forum. You really don't need to use places like MySpace, Facebook, or Twitter, and be forced to deal with privacy management. Closed groups make privacy a lot easier.
But if you choose to join a social network, be social! Take some time to learn how the network shares private information and how you can control that flow of information then get out there and share. Expand your horizons, take a risk and open your social circle. Don't be paranoid and you'd be surprised what might happen.
Tech superstar Robert Scoble recently published a Cinchcast about transparency on social networks where he says his transparency has enriched his life. He's made new friends, gotten business deals, and met interesting people that he may never have met had he kept a closed social group. Scoble is so social, in fact, that he even publishes his cell phone number on his profiles and invites anyone who wants to ring him and talk. As far as I know, his identity has never been stolen, his house has never been broken into because someone saw on his profile that he wasn't home, and nothing really bad has happened to him.
The issues we hear with social networks usually boil down to user ignorance. If you join something that has the word 'social' in it and expect it to be totally private by default, you're being naive and short sighted. It's kind of like choosing not to read the user agreement when you join a new service then complaining that they 'tricked' you when they do something you don't like but was covered by the agreement. The only person responsible for your privacy is you. Not Facebook, not Twitter, not MySpace. They don't even really owe to provide you with privacy management tools. They could just say 'everything you post is public, if you don't like it, don't join' and the responsibility would still fall on you.
It's time people stop bitching and whining about how these social networks are invading our privacy. By choosing not to familiarize yourself with the privacy tools provided and posting stuff you'd rather keep secret, you are voluntarily relinquishing your privacy through your own ignorance. Quick complaining about it and learn how to protect yourself.
Lastly, before anyone starts screaming 'what about the children", well, that's your responsibility too. Social network should make reasonable efforts to keep children safe but, in the end, it's your job as a parent to monitor your child's online activities. Can't be with them all the time? Install software that can. Don't know how to do it? Hire someone to do it for you. It's really not hard to protect your child online at all. You just have to take the time to learn the tools and care enough to use them.
As with anything in life, personal responsibility is the key to safety online. Learn it, use it. Be safe.