Draft schema for Twitter type followers, following functionality:

create_table "followers", :force => true do |t|
    t.integer "leader_id"
    t.integer "follower_id"
end

add_index "followers", ["leader_id", "follower_id"], :name => "index_followers_on_leader_id_and_follower_id", :unique => true

Just an idea, haven't tested this yet.

Update: railscasts.com has published an article on how to use self-referential associations for this purpose: http://railscasts.com/episodes/163-self-referential-association