What's the worst thing that can happen? Insurance companies would love to have access to your social network data -- there is already strong research data showing a correlation between your friends and various risk factors. We can assume that governments already have access to the data, but there is a gradual drift to making this available to other corporate entities. We have the most to fear from commercial entities here.
Now, one could argue that through financial records and other online records, anyone from a private investigator on up would be able to determine everything about me given enough time. I grant this is probably true. The downside by sharing is that every large entity will know everything about me, with minimal effort.
I won't agree to share my information merely to gain a convenient service, in return for the data being mined. This sharing lowers the bar required for corporations to intrude into our identities, into who we are. And it's being performed en masse. In a sense, we're merely data points.