Remember that with cause and effect you want to prove causality and not just correlation. Cause and effect requires a direct connect. This happened because of that and nothing else. Correlation shows that these two events are connected but it is vague on how. A correlation might also prove a relationship but be unable to eliminate other factors playing a vital role in the effects. For instance, Facebook may have led to people spending less time face to face or it may be that society simply moved in that direct at the same time Facebook appeared. You want to prove how the cause generated the effect.
I would suggest brainstorming on the topics you have already singled out. Can you prove there is a direct link between the cause and the effect? It will help to narrow your topics before seeking a cause and effect relationship. For instance, Facebook's effect on society is a very broad topic. You may want to focus on one aspect of Facebook's effect such as changes in communication or changes in the definition of friendships and relationships. The more narrow your topic, the easier it will be to prove a true cause and effect relationship. A broad topic can lead to confusion and doubt. Specific facts and evidence will be needed to go beyond correlation and into effect.