Are social networking sites ruining teenagers (and adults) social lives?
You could make the argument in either direction. Social networking sites both enhance and ruin "social lives".
It’s hard to say something called social networking ruins someone’s social life. Facebook and Twitter help you keep in touch with people that you might not keep in touch with otherwise. They facilitate meeting up and staying in the loop of current events.
However, with Facebook and Twitter, people are less inclined to meet in person and they are more likely to online chat, etc. taking out the human aspect of relationships. This is probably just a reflection of the times of online dating and social networking taking the human-to-human interaction out of everyday life.