Internet is not free, and social networks daily visits either, since you pay with your personal data. This is a message that takes several years trying to instill in society. Many people are aware of it, and even justice is starting to act where services like Facebook or WhatsApp, but this is just the tip of the iceberg.
Something as seemingly innocent as an extension for your browser also may be getting and selling your personal data. Not only extensions of low quality, but, as has uncovered a German environment, also other with such good rating as the popular WOT: Web of Trust. So do not be afraid to how Facebook can abuse their cookies, because surely already have given your details to whom we least expect.