Not that I want to be the dictionary nerd, but I will do it: "the exclusive possession or control of the supply or trade in a commodity or service."
Facebook does not have a monopoly to its user-base. Since we as users don't pay for it and are not obligated to use it, nobody can say the control the supply of ??? social media access? We can go anywhere else to communicate with people and we can post cat pictures elsewhere as well.
Where they may be a monopoly is to the people who pay to access our data and advertise. Except that I don't believe they are there, either. There are many places to advertise on the internet and Advertisers can get data from other sources (Google, for one). Now, as far as directly funneling our personal information to bad actors: nobody does it better.