Nice query to get my friends
It can be done with a simple join (assuming the (user_id, follower_id) combination has a unique constraint):

SELECT 
    f1.followed_id AS friend_id
FROM
    follow AS f1
  JOIN
    follow AS f2
      ON  f1.user_id = f2.followed_id
      AND f1.followed_id = f2.user_id
WHERE
    f1.user_id = 36 ;