An argumentative essay, in my experience, really necessitates that you know enough about your subject to be able to formulate a substantial opinion. A thesis statement, for any form of essay, is basically a concisely phrased argument or message that you intend to support throughout the essay. Therefore, a thesis statement for an argumentative essay on social networking really requires that you know why an argument supporting social networking is a topic at all.
Consider it this way; if you have to support social networking, that means someone is opposed to it. Who would oppose it, why would they oppose it, and what evidence would they use? Based on my personal knowledge (again, if this is a subject that is unfamiliar to you, you'll need to do some research here) I would suspect that people opposed to social networks might include parents, therapists and psychologists, educators and white-collar business managers. Their main arguments would be that social networks are an uncontrolled, addictive, potentially destructive and distracting competitor for a person's social time, lumped into the same category as video games or alcohol. As a teacher myself, I can tell you that many students (and adults) appear to be unable to self-monitor their use of social media, resulting in excessive time spent using it, as well as what, to me, appear to be extreme abuses or oversights of common sense, such as posting sexual or drug/alcohol related material.
As evidence, these opponents might point to the numerous incidents of students committing suicide following cases of "cyber bullying" via social networks, or of employers losing money due to employees wasting time online (see my source below).
A good thesis will address these arguments, give evidence or reasons they are inaccurate, and assert its own position as a superior solution. For example, you could go point-by-point, trying to find evidence which refutes these arguments, or you could make an argument which circumvents these concerns. Take the Arab Spring, which was in part fueled by social media; many people would agree that the benefits of a social network, in this case, outweigh our domestic concerns about lost productivity. A thesis for this argument could be "The use of social media as a political, diplomatic, or even military tool easily compensates for problems arising from its domestic (mis)applications."