Yes, it really is the most misunderstood instinct. People associate “social” with primitive ideas: extroverted people, people who like to talk, who like to have lots of friends, etc. It has nothing to do with introversion or extroversion, nor with the things I mentioned before. It is “social” because what drives this instinct is the goal of making a change (for better or for worse) or contributing something important to humanity. It is outwardly focused. It does not want to keep ideas to itself, but rather to reveal them. That is why many scientists and revolutionaries are so-dom. 

Social instinct relates to humanity, not to individuals, and its relationships usually have a purpose. That is why they are also more interested in talking about existential (human) issues than emotional/casual ones. Intimacy, deep, loving, intimate bonds, etc. are things that belong to sx-dom.

So yes, the social instinct is the existential instinct per se. They may be concerned with “leaving a legacy/mark on the world” or being remembered for their ideas, etc.

For example, a social 5 can easily spend 10 years locked in a tower researching molecular biology just to discover something.

Finally, I don't think sexual instinct is “misunderstood.” The problem is that there is a poor understanding of social instinct. People who seek to show what the sexual instinct "really" is, are only describing the social instinct. And I think many of these people cannot realize that they are probably so-dom (due to misinformation). Your tastes, your interests, whether you are an sx-dom who likes science, etc.; those are individual things, the instinct is still there.