Is PostgreSQL an easy choice for a large CMS?

Hello all,

I'm working on a CMS which requires an open source database capable of
handling hundreds of thousands of users simultaneously, with a high
rate of database writes, and without buckling. We're talking somewhere
between nerve.com/catch27.com and xanga.com/friendster.com

PostgreSQL is a personal favorite of mine, and my gut instinct is that
it's the best choice for a large scale CMS serving many users;
however, I'm getting antsy. I keep getting suggestions that Postgres
is really only suited to small and medium projects, and that I should
be looking at MySQL for a large scale database drive site. I'm not
really a fan of MySQL, but I'll consider it if it truly is the better
choice in this case. I just don't understand how it would be. I'm
thinking this is solely in reference to VACUUM. Even with autovacuum
suport, I tend to agree there is at least one handicap.

I could really use some enlightenment on just where PostgreSQL fits in
a single-server, highly-trafficked web site serving mostly text,
pictures and possibly streaming media.

Regards,

Anthony