These companies are in business for themselves, they don't have much incentive to protect their customer's information (except that if they screw over enough people, then they will get bad press.)
The concern is that any information you upload to their site is now available for them to use in whatever way they choose. Maybe not now, but someday a guy in their Marketing dept will figure out a way to data-mine their databases, or establish some kind of trends based on the information they have. I'm not completely comfortable with that.
Same concept social networking sites such as Facebook, any pictures that people upload to their servers are now the property of the company who owns the site. That's one reason why I dont have a Facebook account.
Also, companies pay them for access to member profiles so they can search for employees. But you are right, they have information you share with them. I wouldn't be surprised if they sell user preferences to marketing firms or something of that nature. Anyway, I'm not so paranoid about it, I can use it to maybe find a job out of school or who is hiring what types, and I'm fine with people knowing about me what ever I choose to share with the website, if I wasn't I wouldn't share it. Facebook I find a lot more intrusive and seldom use, it makes everything too public and wants to track you on your phone and what not.