It sounds like you're talking about things like Facebook and Google+, which are really just communication networks, similar to phone services and postal services. We don't allow postal or phone services to look through our communications and then compile and sell that information to 3rd parties, so why should we allow Internet communication services to do that? It's not just networked messaging sites either, Gmail and Yahoo Mail scan through the E-mail messages we send and receive, looking for keywords that they think will tell them something about us that will be valuable to advertisers, who, not surprisingly, are also our employers and service providers. People who communicate over the Internet deserve the same sort of protections that people who communicate over phone lines or through the post receive.
Lessismore posted about some tracking here on Hubski that raises some related issues. I'd like for you to weigh in if possible: http://hubski.com/pub?id=7144