General sense: a person who follows a leader, movement, or ideology. Less common than the social media sense in modern Japanese.
I became her follower.
He is a charismatic leader and has many followers.
Specifically refers to a follower on social media platforms such as Twitter, Instagram, etc. This is the most common usage in contemporary Japanese.
My Instagram followers exceeded 10,000.
I post every day to increase my followers.
新しい動画でフォロワーを増やした。
He increased his followers with the new video.
He has 100,000 followers.
This campaign gained a lot of followers.
支持者 stresses active support for a person's views or cause, whereas フォロワー just means someone who follows, without implying agreement.
フォロワー is someone who follows, the direct opposite role to リーダー, the one who leads.
followership
Borrowed from English 'follower'.