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.
インスタグラムのフォロワーが1万人を超えた。
My Instagram followers exceeded 10,000.
フォロワーを増やすために、毎日投稿している。
I post every day to increase my followers.
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.
フォロー is the verb/act of following someone online, while フォロワー is the noun for the person who is doing the following.
The everyday counter for people: 一人 (ひとり), 二人 (ふたり), 三人 (さんにん).
More formal than 人 — used for headcounts in reservations, staff lists, and announcements (5名様).
followership
Borrowed from English 'follower'.