A social misfit is generally thought of as someone who has different views on culture and doesn't fit in with most others in society. In recent years, the term social misfit has expanded to be used as a slur against people who don't have a lot of friends, or who find it difficult to get acclimated to a particular group.
The problem with this term is that it implies that anyone outside of the normally accepted boundaries of friendship or social interaction is weird or odd. The reality is that a true social misfit chooses to be held apart from society in some way, as in rarely going out in public or refusing to have friends. While some may struggle to make or keep friends, it doesn't mean they are misfits of society.
Another point to consider when using the term social misfit is generally accepted societal norms. These are commonly accepted practices about blending in with the rest of the people around us in our schools, workplace, or community. In some cases, norms have been created over time to protect us, help us with etiquette, or even to help us relate and connect with each other.