Counting different strings (OK%, FB%) in same table, grouped by week number

Hello,

I have a table holding week numbers (as strings)
and user ids starting with OK, VK, FB, GG, MR, DE
(coming through diff. soc. networks to my site):

afarber(at)www:~> psql
psql (8.4.9)
Type "help" for help.

pref=> select * from pref_money;

id | money | yw
-------------------------+--------+---------
OK19644992852 | 8 | 2010-44
OK21807961329 | 114 | 2010-44
FB1845091917 | 774 | 2010-44
OK172682607383 | -34 | 2010-44
VK14831014 | 14 | 2010-44
VK91770810 | 2368 | 2010-44
DE8341 | 795 | 2010-44
VK99736508 | 97 | 2010-44

I'm trying to count those different users.

For one type of users (here Facebook) it's easy:

pref=> select yw, count(*) from pref_money
where id like 'FB%' group by yw order by yw desc;

yw | count
---------+-------
2012-08 | 32
2012-07 | 32
2012-06 | 37
2012-05 | 46
2012-04 | 41

But if I want to have a table displaying all users
(a column for "FB%", a column for "OK%", etc.) -
then I either have to perform a lot of copy-paste and
vim-editing or maybe someone can give me an advice?

I've reread the having-doc at
http://www.postgresql.org/docs/8.4/static/tutorial-agg.html
and still can't figure it out...

Thank you
Alex