I suspect that the same is true of other services, like Google+, Twitter, Myspace, Blogger, YouTube, and Wordpress, but the complexity of activity that can be searched may generally be less with these sites.
The Boston Phoenix "Phlog", an alternative newspaper, has researched this capability with the "Craigslist" incident in New England (subject of a Lifetime movie about Philip Markoff in January 2011), link. The "Phlog" has a redacted pdf of the server log sent to the Boston Police Department.
As I noted Sunday, online publishers have varying policies regarding knowing who visits their sites, since the practice is not accepted in the print world. But server logs can often provide "valuable" information about "who" looked for "what" information on a site with specific (perhaps troubling) search arguments.
Furthermore, Business Insurance (April 2, 2012) has a story by Judy Greenwald advising employers not to ask for social media passwords, and reporting that there is an effort in Congress and some state legislatures to outlaw the practice, which is infrequent but increasing. The same issue of the same publication has another article by Sheena Harrison, p. 3, "Risks from social media, ACO's complicate medical liability", here. Online the title has a different spin, "Social media policies can reduce risk of reputational damage".
Picture: Union Square, demonstration, NYC, March 24.