In many ways, Facebook does the exact opposite of providing privacy. Instead, it puts any personal information that a person uploads in front of hundreds of friends and associates. However, there are still many ways in which Facebook is supposed to keep sensitive information private, and a lawsuit was started against the company for failing to do so.
One way to communicate on Facebook without exposing those communications to the world--which is what happens when writing on someone's "wall" or "timeline"--is by sending private messages. No one is supposed to see those messages except the sender and anyone that the message was sent to.
However, according to the lawsuit, Facebook violated this privacy by scanning the messages without telling any of the users about it or asking permission. This doesn't necessarily mean that all of the messages were read, per se, but that the information in them was harvested so that Facebook could determine what people were talking about.
Though Facebook does own the site, of course, this was still seen as a violation of the privacy of the users, who did not know that what they said would be analyzed by a third party.
So, why did Facebook do this? Their reasoning, the lawsuit claims, is that they were concerned with advertising. The company wanted a larger presence on the web and more advertising money, and getting as much information as possible about what users were interested in was one way to do it.
With large-scale cases like this, it's important for all people in Canada who may have been impacted to know their legal rights.
Source: Rochon Genova, "Facebook Privacy," accessed April 08, 2016