The answer to this question depends on how you use social networking sites and what kind of experiences you have had. I normally use the sites just to browse and keep up with what's going on in certain people's lives and to keep up with certain events. If a person uses social networking sites as their primary form of expressing themselves and communicating with others, then I would say in those instances, social networking sites have not been good for society.
This question is largely a matter of personal opinion. If you need to write a paper on it, you will need a thesis. I will argue that social networking is not good for our society.
Social networking is not good for our society, because it only approximates true community. There are several reasons why this is so. First, social networking is usually digitally based, such as when people use facebook. So, there is very little personal contact. In a word, having a virtual friend is no friendship at all.
Second, social networking is all about numbers. You want to reach as many people as possible, this creates for shallowness of community.
Third, social networking is harmful, because it gives people the sense of community when in fact, there is little of it. This delusion can persist for a while and in the process make people inept at real relationships.