Does anyone here have feet in both camps? I'd like to read that person's opinion.
I would trust Postgres for any production application -- however, Oracle has a few nice solutions when it comes to business intelligence that I haven't found for the free databases. Further, so many solutions (such as the major ERPs) come on big hardware databases that mid-sized companies already have the necessary database. Why use Postgres when you already have a license (or three, including development and staging/training) for a commercial database?
Certainly, at the Fortune 500 level, Oracle and DB2 have advantages that are YAGNI for us. (For example, Oracle RAC is a pretty elegant solution that isn't right for serving Facebook, but certainly adds more convenience than it costs in complexity for a large company.) You can have a big cluster of databases and use a single point of control across the cluster. I just haven't seen the same tools for Postgres.
I would never suggest that Facebook should be served up by Oracle. It just wouldn't make sense considering the cost. However, for a large company with a dozen (or many more) large interrelated applications, both consultingware and internal, used on three continents? I wouldn't want to manage a Postgres cluster....
(I am, of course, oversimplifying things.)