Why facebook used mysql ?

There was an interesting post today on highscalability -
http://highscalability.com/blog/2010/11/4/facebook-at-13-million-queries-per-second-recommends-minimiz.html

The discussion/comments touched upon why mysql is a better idea for Facebook
than Postgres. Here's an interesting one

> One is that PG doesn't scale that well on multiple cores as MySQL nowadays.

Another is in fundamental differences of storage architecture - all
> MySQL/InnoDB data is either a clustered primary key, or secondary key with
> PK pointers - logical relationships between entries allow to have index-only
> scans, which are a must for web-facing databases (good response times, no
> variance).

One more reason is that in heavily indexed databases vacuum will have to do
> full index passes, rather than working with LRU.

As for sharding, etc - there's no way to scale vertically infinitely - so
> the "stupid people shard" point is very very moot.

It is much cheaper to go the commodity hardware path.

or

In general Postgresql is faster at complex queries with a lot of joins and
> such, while MySQL is faster at simple queries such as primary key look up.

I wonder if anyone can comment on this - especially the part that PG doesnt
scale as well as MySQL on multiple cores ?

regards
Sandeep