In my experience, people do not always "instinctively leave certain tidbits" off of Facebook - in fact, people often feel like Facebook is in many ways a private space. Like "it's just my friends," even though the friends and friends of friends circles are massive, not to mention privacy controls that make many things public by default. Also, FB prompts for things like employment history and educational history in the same way as LinkedIn, but goes further to ask for personal info, geographic history (where all have you lived), etc.

Combine all this with the fact that there are so many FB users and many of them are children who wouldn't otherwise have a LinkedIn account (and are more comfortable with sharing personal info online).

So, while I agree that LinkedIn is in the group of data collectors, I don't think they're quite on the scale of FB.