"Social" has to do with human relationships and groups of people within a society while "societal" has to do with society as a whole, as a system. The use of "societal" is more restrictive and academic (it's a social science term). You'll rarely hear it in general conversation or even on the evening news. Substance abuse is a social issue in the sense that it negatively affects an individual's personal relationships and professional life. But it becomes a societal issue when society as a whole is at risk because of it. Street violence, a climate of insecurity (due to drug wars), reduced productivity and increased health care costs are various societal effects of substance abuse.

The word social has many meanings, but common ones are something like: 1) pertaining to friendly companionship 2) seeking to enjoy the company of others Societal, on the other hand, is something more along the lines of: 1) referring to large social groups, or to their activities and/or customs

As you probably know "social" has a wide range of meanings: social networking, social media, social event (eg a party), a social person (someone who enjoys being around people and making new friends). It also can be used to describe things that relate to society in general: a social problem, for example, is an issue that affects most of society or is caused by something in society, for example gun violence in the US. "Societal" has a very narrow range of meanings. It pretty much only has one meaning, which is similar to the last meaning of "social" given above. The Oxford Advanced Learner's Dictionary defines it as: "connected with society and the way it is organized." It is a technical word for academic writing, not something we use in conversation very often.

Social means one who can communicate well with others, but societal means "Partnership for entrepreneurs, society." (according to Google) It appears to be a Sicilian word also.