You might not post private information to Facebook but when and how often you post can tell someone more than you would casually share with a stranger like what your sleeping habits are or if you are at a special event (determined by the lack of generated events during a normally scheduled period of activity for you). After a while, even the non-interactions with the site can tell the site and their advertisers more about you than you might prefer.
Think of twitter bots like the automated RobMeNow tweets. Sure letting people know that you are buying a subway sandwich is mundane and hardly private information but entering that information into an archive that can be analyzed to determine how much you spend on fast food and when you are most likely to purchase fast food is probably not something you want to share with advertisers.
EDIT: People are commenting about how it's not so bad that advertisers know this information about you. I just want to add that you can replace "advertisers" with any noun that is legally or commercially entitled to the data. It could be a government or the company that acquires your social network.