I wouldn't mind sharing more, in some cases. However, we do not have a -- codified -- social convention for how such information can and cannot be used. When I can lose health insurance, or a job, or -- in other countries -- potentially my life, as a result of public sharing, I'm going to avoid it.

Facebook grew to dominance in good part because it initially provided people control over their sharing. Statements now that they "give users (complete) control" over how their information is shared, are disingenuous. Lies, actually.

You can no longer hide your profile picture. Many of my friends have replaced theirs with photos of their pets, random graphics, or whatnot. This occurred after profile pictures -- even pre-existing profile pictures uploaded and selected at a time when they could be limited to e.g. just friends -- were forced to be public.

As someone who has had a family member selected and stalked based on appearance, I can tell you how troubling this is. Not to mention all the potentials for discrimination of various sorts. Yes, you can change your picture, but can you ever change the old version that Google and other sites cached before you caught on to the site's change of its visibility?

You cannot hide your friends (unless this has changed in the latest changes, for which I've not yet been prompted by the site). Even more insidious, there is a workaround that allows you to remove the "friends" widgit from your public profile page, but which does nothing to limit the availability of your friends list through the invisible, much less apparent, and much more commercially oriented API's.

Why should I worry about all this? Well, this article provides a decent, if thin on specific incidents, summary:

http://news.ycombinator.com/item?id=1387385

Many things are "easier" when you are young and don't have much in the way of obligations (particularly in U.S. and some other Western/"advanced" societies). Or when you are a "superstar" and many people seek your attention and participation -- and are willing therefore to overlook some "eccentricities". A lot of people, however, are not in either of those positions.

Look how much trouble the U.S. has just trying to reach some consensus on how to deal with pre-existing medical conditions and health insurance. This despite the fact that the majority of the population seems at least to profess an aversion to, more so an abhorrence of suffering. And despite the fact that one can pay into insurance plans for years and years -- what might under other circumstances be interpreted as building up a considerable, as yet unrequited investment in the system. And yet ultimately be denied coverage due to a job loss, or just to the greediness of a system designed to place executives and shareholders first.

Comment on Facebook on your sore back or that RSI, and while you may receive some sympathy or even helpful suggestions in the short run, our current society and legal system does nothing to prevent that data from being mined and then used one day e.g. to deny insurance coverage.

Why are some people upset about privacy? Because some people have paid attention to the discriminatory uses and misuses of information, either in their own lives or in others', contemporary and historical.

The novel physical aspects of the Web can seem to cast this in terms of a new, unexplored context. But much of the underlying situation is neither novel nor surprising.

Personally, I am actually not a shirking violet in terms of sharing personal information, even with strangers. (Sometimes, in fact, it is easier to share things with a stranger; also, the novelty of getting to know someone is an often very enjoyable experience.) But I do NOT want it absorbed and ensconced into a medium that works mercilessly to associate it -- publicly -- with my identity and continues to hold it over my head for the rest of my life (in ways that literally threaten my well-being).