A child only begins to talk if his parents speak to him.A person picks up the accent of whatever area they are in.People are influenced by other people.If your social circle has bad health habits and you are associating with them, you will pick up those bad health habits as well.
Social interactions may influence a person's choices as far as diet and exercise but it's not an absolute determinant and it won't effect the person's health more than how much they actually eat properly and exercise.
Although on average people will tend to adopt the health habits of those they associate with it's not absolute or inevitable. A person can bare in mind that just because people around them are doing unhealthy things it doesn't mean they have to too. You can resist peer pressure.
Longevity is hardly effected by social interactions. There are many higher functions that are directly linked to having positive social interactions, but most people live longer because they eat right and don't sit around all day. The number one killer in the US today is heart disease. This 'disease' is completely preventable by leading an active lifestyle and eating healthy. That leads to better bloodflow and a stronger heart. Less probability of the leading cause of death is quite more substantial than having a few friends over for scrabble.
No, I do not think that social interactions are strong determinants of longevity and health than diet and exercise, because they help a person live longer. Some people who live to be very old are extremely mean people. Keeping healthy habits are more important than laughing once in a while.
I do not think that social interactions is a detriment to your health at all. Neither is diet and exercise. Both are very important to a person's overall health. What's detrimental is not having a social life and not having a healthy diet and fitness routine. It's all about balance in life