There are a lot of people who may think that Facebook and LinkedIn are just two similar websites that will allow people to be connected to each other but actually, they have some differences. Facebook is more used for casual purposes. You can connect with your classmates that you have not seen for a long time. LinkedIn is known to be more professional.
You may be connected to each other because you have common interests, you are potential business partners, you are co-workers, and so much more. It is highly possible that you have a more professional relationship with a person when you are connected on LinkedIn.