Para isso precisamos filtrar os dados no select e não no join.
Remova esta condição:
AND (month(d.date), year(d.date)) = (month(CURDATE()), year(CURDATE()))
e adicione isso ao select:
SUM (CASE WHEN (month(d.date), year(d.date)) = (month(CURDATE()), year(CURDATE())) THEN 1 ELSE 0 END) as monthly
Editar:
consulta inteira:
SELECT users.id, users.username,
COALESCE(sum(CASE WHEN (month(donations.date), year(donations.date)) = (month(CURDATE()), year(CURDATE())) THEN donations.amount ELSE 0 END), 0) monthly_sum,
COALESCE(sum(CASE WHEN (month(donations.date), year(donations.date)) = (month(CURDATE()), year(CURDATE())) THEN 1 ELSE 0 END), 0) monthly_amount,
COALESCE(sum(donations.amount), 0) total_sum,
count(*) total_amount
from users
left join donations
on donations.uid = users.id
group by users.id, users.username
http://sqlfiddle.com/#!2/20a974/20/0
