Su consulta actual solo mostrará un resultado si todas las tablas tener un enlace con la mesa de tus amigos. Creo que buscas más algo como esto
SELECT following
FROM friends
WHERE follower='id'
UNION ALL
SELECT following
FROM family
WHERE follower='id'
UNION ALL
SELECT following
FROM following
WHERE follower='id'
UNION ALL
SELECT following
FROM acquaintances
WHERE follower='id'
o un poco más agradable de leer y más fácil de ajustar a costa de algo de rendimiento
SELECT following
FROM (
SELECT following, follower FROM friends
UNION ALL SELECT following, follower FROM family
UNION ALL SELECT following, follower FROM following
UNION ALL SELECT following, follower FROM acquaintances
) AS f
WHERE follower='id'
