Hello Naadem, there are multiple areas available for research in social networking - if you already admitted to the PHD program you should check with professors if there are specific areas of
research where they need your assistance. if you are working on your undergraduate thesis - check with the PHD program coordinators.
My personal preference would be problem of spam and trolling in social networks and trends in these areas, but you should decide this on your own based on which background and goals you have.
HTH, Vlad.