As always the issue here is not the type of information (data valuable to divorce lawyers) but the context in which it is gathered (Facebook search unbeknownst to the poster). And once again the usual responses will be - a) Poster is stupid, and b) Facebook is evil.
I tend to think that so long as you are empowered to share or not to share then all is well. With Facebook this is not the case. My sister shared a reasonably embarrassing photo of me with some mutual friends (some of which I work with) which was then shared with my whole building by whatever networking effect took over - nice!. I was not in control of this. Now you can argue that she could have done this pre-social networking site era - but she couldn't simple because she is not in physical contact with 99.5% of people in my building. Social networking makes ones dis-empowerment that much more pervasive.