The top story on many tech news sites today is Facebook’s most recent “about-face” decision late last night to change some of the features of their new Beacon advertising program, which shared details about members’ purchases and other off-Facebook activities without providing an easy way to opt out.

But some observers argue that Facebook users, who routinely and willingly share many personal details with their friends, don’t have any grounds to complain about breaches to their privacy, calling them “hypocritical” and “willing to give up tons of privacy information for like, free crap.”

What seems to rarely get mentioned—perhaps because we haven’t yet found great ways to articulate the user experience—is that the choices people make to share personal information on social networking sites and other places online are highly contextual ones (see our discussion of this issue with respect to teenagers’ use of social networking sites here).

Setting up an account is but one very small step in managing an ongoing, dynamic relationship with a network. Decisions about what you are willing to share, or what you have shared in the past get weighed against a complex set of ever-changing factors—growing audiences, evolving relationships as well as shifting privacy settings.

When you create your account, you might post a fun, playful photo of yourself sipping cocktails in a cowboy hat with friends in mind, but when colleagues start to enter your network, you change the photo to something that’s more reflective of your professional identity.

As we have found in our research, teenage users of social networking sites often withhold certain pieces of personal information, particularly in the public portions of a profile. However, it’s often only through a messy process of trial-and-error that users with large friend networks realize that when private networks become big enough, they can start to feel very public.

Realizing that you’ve inadvertently shared information about buying a nose hair trimmer for yourself with 75 “friends” in an overlapping network of family, neighbors, and co-workers could be one of these messy moments, for instance.

Ultimately, we are all still learning as we go about the constantly changing nature of participatory Web applications—cowboy hats and all.