Every fan of yours can become a top fan if he/she meets at least on of the following criteria: having a high performance and/or high influence.
When a fan has high performance it means that he/she has a lot of contact with your organization. If you want to know more about the contact you had with one of the fans, you can easily look him/her up via the CRM search.
When a fan has high influence it means that he/she has a lot of friends or followers. So getting in touch with this fan will be seen by all of his/her contacts, bringing your brand to a lot of (new) people.
Do note that the number of contacts/followers gets imported directly from the channels. However, Facebook doesn't allow this so this results in Facebook showing up as a channel with low influence.