Lead with Postgres

gistfile1.sql

select months.month, 
	COALESCE(users.count,0) AS count,
	lead(count,1) over (order by months) as prev_month,
	(count - (lead(count,1) over (order by months)) ) as delta
from
(
	SELECT 	generate_series(
			date_trunc('year', min(created_at)),
			now(),
			'1 month'::interval
		)::date as month
	from users
) months
INNER JOIN
(
	SELECT 	count(*) as count
		, date_trunc('month', created_at)::date as month
	FROM   users
	GROUP BY date_trunc('month', created_at)
) users on users.month = months.month
ORDER BY month 


month           count   prev_mondelta
"2009-04-01"	4	27	-23
"2009-05-01"	27	21	6
"2009-06-01"	21	28	-7
"2009-07-01"	28	30	-2
"2009-08-01"	30	20	10
"2009-09-01"	20	27	-7
"2009-10-01"	27	19	8
"2009-11-01"	19	25	-6
"2009-12-01"	25	33	-8
"2010-01-01"	33	17	16
"2010-02-01"	17	30	-13
"2010-03-01"	30	23	7
"2010-04-01"	23	26	-3
"2010-05-01"	26	23	3
"2010-06-01"	23	28	-5
"2010-07-01"	28	34	-6
"2010-08-01"	34	19	15
"2010-09-01"	19	28	-9
"2010-10-01"	28	26	2
"2010-11-01"	26	19	7
"2010-12-01"	19	25	-6
"2011-01-01"	25	26	-1
"2011-02-01"	26	27	-1
"2011-03-01"	27	28	-1
"2011-04-01"	28	24	4
"2011-05-01"	24	25	-1
"2011-06-01"	25	20	5
"2011-07-01"	20	18	2
"2011-08-01"	18	25	-7
"2011-09-01"	25	23	2
"2011-10-01"	23	25	-2
"2011-11-01"	25	22	3
"2011-12-01"	22	17	5
"2012-01-01"	17