The latest is over Facebook facial recognition, which can automatically tag friends in photos just by matching the image to a massive database of faces.

Face recognition is a useful, time-saving feature -- at least . But it's also a creepy addition to Facebook that opts you in automatically. As my colleague Sarah Jacobsson Purewal reported, of getting automatically tagged by friends. The database can still technically match your name to your face.

Therein lies Facebook's big dilemma, the one that comes up time after time, with each new change to the site that demands more of users' personal information: Yes, letting users opt-in to new features would be a more respectful approach. But because Facebook is inherently social -- that is, it relies on the participation of many users -- opt-in is much trickier to pull off. In some cases, it's just impractical.

Take, for example, the "" feature introduced last year. This allows partnering Websites to use and display information from your public Facebook profile, and from your friends' public profiles. For example, if you write user reviews on Rotten Tomatoes or Yelp, your friends can see those reviews when they visit the site, provided they're logged into Facebook. Had Facebook made this feature opt-in instead of , most people wouldn't have bothered. That would defeat the purpose of personalization, which relies on having lots of recommendations from people you know.

A simpler example is Facebook's broader attitude toward public vs. private information. In late 2009, Facebook made changes to its to put an emphasis on "everyone," so that users would share their status updates with the entire Internet by default. In making this change, Facebook was trying to be more like Twitter -- a massive, ongoing, public conversation between lots of people, regardless of whether they're friends or strangers. I like Twitter, and I understand by Facebook would want to make this change. But again, it only works if a critical mass of people are participating. That's why the "Everyone" option for status updates is opt-out, rather than opt-in.