Writing papers can be frustrating, but you are well on your way. You have a great thesis: "Facebook is a threat to our society as it can have negative impacts on our relationships, compromise our safety and minimize our privacy."
You also have your supporting point, which I think are great. So, in the first paragraph, write you thesis down. You can even do something simple as writing, "This paper will argue that Facebook is a threat to our society in many ways." After this you would write your supporting point. This is your roadmap.
The next three paragraphs will be devoted to your supporting points, namely that Facebook impacts our relationship negatively, compromises our safety, and minimizes our privacy. Each of these body paragraphs will, then, have to be developed more. You can give examples of each negative impact. That would be a great place to start.
Finally, the last paragraph will be your conclusion. Summarize your findings. Best of luck.