I recently did a social network analysis for a membership association as part of a presentation on "professional vs. personal networking." (E-mail me for a copy of the slides.) Afterwards, the coordinator of the group commented how quite of few of the most talented networkers in the room were precisely those who did not return their surveys. Whether personal or professional, relationships are privileged information, and responsible social network analysts must respect those who choose not to divulge. For an in-depth discussion of SNA ethics, see Borgatti and Molina's "Toward ethical guidelines for network research in organizations."
In stark contrast to those secretive networkers, many other seemingly judicious people become blatant exhibitionists in their online personas. Wendy McClure's essay "Mysteries of the Amazon," which appeared a week ago in the NY Times, is a highly entertaining confessional about the awkwardness that results when secrets meant for e-strangers fall into the wrong hands (in her case, because she is a digital snoop on her friends).
In regular life, we have developed sophisticated strategies for when to share which parts of our life stories. The online version of this is far from being worked out, but impressive progress is being made. For a look at the leading edge, see the Eclipse Higgins Project, led by Paul Trevithick of Parity Communications.