Q: Is it ethical to study people in cyberspace when they don’t they are being studied?
The answer to this, save for some exceptions and caveats, lies in the means used to procure the information. Doing a case study of individuals on a public forum is, in my opinion, completely acceptable as these people are putting forth their opinions and perspectives in a open setting. They may not have explicitly stated that their posts might be used for research, but their willingness to put their thoughts out in such a way implies that there is little concern for privacy.
On the other hand, data mining individuals’ web surfing habits against their knowledge for use of a study would be a violation of a reasonable expectation of privacy. If individuals would like to share what websites they frequent the most and why, it is then absolutely acceptable to use the information. Google selling their compiled search information from all their users to research groups would be ethically shaky at best; this is assuming the users did not know of it and they likely never would in a situation such as this.