If you want to refer more then you go to library or you have any books relating to this topic you can refer or else best thing is you type in google that essay on - social networking sites are valuable, you type like  this you will get more information.
                                         AND I WISH A VERY ALL THE BEST FOR YOUR DEBATE COMPETITION.

plzzzz mark as best and say thank u
Basically if u r going for the topic the points u could focus on r
it helps us connect with those who r far away in a much faster and easier way at the click of a button
social networking sites have also helped in the rise of globalization
social networking sites like edmodo can be used as valuable study materials where the teacher and students can share info on a private group.

and u could get a few more points and round it off by saying
social networking sites r valuable to us in many ways but only in the right hands(cause some ppl misuse them and all)
if u go against the topic there r plenty of sites on the internet stating the medical and social cons of social networking sites.
I'm sure u could find something similar to this on,it would be really helpful and give u tons of good points.