Killer's Diary Shows Need to Patrol Web: Top News Stories at Officer.com: "Two days after a gunman killed three people at a suburban Pittsburgh health club, local police said they are taking a closer look at monitoring the Internet."
When you post something to the web, you are publicizing it, so I don't see this proposal as an invasion of privacy in the normal sense. It is like reading something you spray painted on a wall that could be seen by anybody who passed by. In Fourth Amendment terms it is what we call "plain view," meaning if police are in a place they have a right to be and see something that has value as evidence, it is usable with no warrant because it isn't really a search at all. This Sodini creep created a web page that was out there, but probably nobody saw it because (duh) he didn't have any friends who cared what he was doing.
On the other hand, this kind of police "monitoring" is a bit troubling because it could lead to a lot of frivolous investigations and reputational damage. People say things on the web that they don't mean and would never say but for the anonymity of the web. Many people take on a persona, give themselves a screen name, and talk like some character they made up. When it gets out of hand, moderators often step in, and that's that. But with the cops looking for potential future mass murderers, that kind of rhetoric takes on a different aspect. The anonymity of the web is illusory because a law enforcement officer can get access to identifying information by a number of means. And suddenly a person could be the subject of an investigation because of something he or she said as "Mr. Doomsday" or some such ridiculous moniker, and never meant at all. He puffs himself up to look like a tough guy, and a cop thinks he is a potential killer.
For example, I have seen posts here and elsewhere in which people say some pretty dire and threatening things about lawyers, property managers, and HOA/condo directors. I have even seen expressions of support for killer Richard Glassel who shot up his board meeting in Arizona, killing two people and wounding several others. He was sentenced to death in 2003.
I hope we don't end up with people getting targeted, investigated, interviewed, and even arrested, for venting their spleen on the web on the grounds that someday they might kill somebody.