The 3 "main reasons" you seek are actually the sub-categories for your argument. You are proving that social networking sites and technology do enhance relationships. Brainstorm. How? How do social networking sites and technology enhance relationships? Make a long list. Write answers that are both general and specific. Include personal examples, observations, and things which you have heard on the news or read about.
Once you have a long list of ideas (more than 10 would be best), you can look at your ideas and see what categories they seem to fit into. Off the top of my head, I'm guessing one main category is going to have to do with the fact that social networking and technology have made keeping contact and building relationships more convenient.
Your other sub-topics are going to depend entirely on the list of examples you personally can brainstorm. If you need more help in the brainstorming stage, I encourage you to post a question in the discussion forum and combine answers from several different people.