If you need to narrow down a broad topic like immigration or electric cars, look at some sources that have already broken down the topic into smaller parts.
You will not be using Wikipedia as a final source, but you can use it to do some early research and to narrow down your topic.
Go to the entry in Wikipedia on social media. How has Wikipedia broken down this broad topic into smaller parts?
You do not have to read the entire page. Look at the contents sidebar or scroll through the page. Notice the headings and subheadings and how they are narrower, more focused parts of the very broad topic. For example, there are sections for Use by Organizations, Use by Individuals, Social Impacts, and Criticism, Debate & Controversy. Notice how even these are broken down into more specific topics such as Use by Individuals > Negative Interpersonal Interactions or Social Impacts > Stereotyping.
Write down the topics that interest you and do more research. You might find that topics become even more focused as you continue to find more information and use other sources.