I have to return to this subject as this came up in discussion with friends. This is a quote from Facebook terms of usage:
"For content that is covered by intellectual property rights, like photos and videos (IP content), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it."
This means that Facebook can for example sell the pictures you post in Facebook to third party. Now, for this I know that law may vary around the world, but around here, there is precedent that if you post a picture of your friend to Facebook and Facebook sells that, and your friend gets annoyed finding his/her face on the bus stop advertising something or another, you are the guilty one in the case. Reason is that you have knowingly posted a picture with purpose to sell (as the terms of usage states that all posted pictures can be sold) and on the other hand, when someone is identifiable/recognizable in a picture, you need permission to sell/use/publish that picture even if the copyright of that picture is yours.
So if the person in picture is after compensation for having his/her face somewhere s/he did not give permit, Facebook is not the one paying but you are. I think Instagram at one point had this same in their terms of usage, but it might (or might not) be removed.