From Ashley Carman, writing for The Verge:
It’s important to understand, however, that the way startups like Slice (which owns Unroll.me) and the giants of Google and Facebook treat your data is extremely different. Google and Facebook analyze information about you gathered through your email, social networking habits, demographics, and location, and use that data to entice third-party companies to advertise with them. In other words, the information they have on you allows them to sell access to your eyeballs for targeted ads in your news feed or sponsored search results.
Startups like Slice, on the other hand, collect information about you and other users and sells it to outside firms. Once it’s handed off, the client can do what it wants with the data. In both cases, however, data is sold in the aggregate, meaning you personally aren’t identified. Although admittedly, compiling a report specifically for Uber feels a little icky in light of recent negative press.
This is good explanation of the the differences between Facebook, Google and other free services. However, the lesson from Unroll.me is that you get what you pay for. If it's free then most likely your data is the product that will be sold.
This is why I'm hesitant to go all in on Facebook as publishing platform. While they do have the audience, I'm not sure I want hand everything over to someone who ultimately is interested in selling my data.