Many of the hosting providers provide PG as a service in multiple clouds / hosting providers allowing you to run your applications on your own VMs in the same cloud. So while you would be using the external IP addresses to communicate with the database it'd still be in the same datacenter.

Also, as pointed out in the parent post, some of the hosting providers such as us (Aiven, https://aiven.io) are directly involved in PostgreSQL development and one of the bugfixes in the latest releases (9.6.1 and 9.5.5) that just came out was contributed by us.

It depends on the type of application and the requirements and expectations of its users. I wouldn't recommend having a web app talk to its database over a WAN, for example. Unless it had a decent caching layer in between.