We use currently use Google Cloud (SQL) and DigitalOcean (Ubuntu 14.04.5) for hosting.
Since most of our users are US based, the servers are hosted there.
As we grow, we will add servers in Europe and migrate to Google Cloud only..
Access to the servers are secured by password protected SSL certificates and whitelisted IP addresses.
Podio data is cached on a secure and encrypted Google Cloud SQL server.
We collect the items of the app where the Portal was installed, and its related items on demand. We also cache items from App View if you have enabled this.
We install webhooks these apps to keep the items up to date in the database.