I really do not recommend that you operate your intranet using a free platform such as Google sites or Facebook page. Security is very important because you would have company information that you are publishing on your page.
If you build an intranet using Google site, meaning your site is hosted in an external platform (owned by Google) in which you may not have complete control of. There may be information that you may not want unauthorised persons to access to.
In my previous companies where I manage the intranet, hosting it Google site or any platform such as wordpress.com (non self hosted site) or a blogspot site would be totally out of the question. There is no way the compliance division would ever approve this.
We would definitely run into audit findings for hosting a site that is meant for internal employees at a free hosting service.
When you are running an intranet, you would definitely need to host the intranet in servers that are totally within the bounds and control of your company, either in-house or secured servers as approved by your company.
In the past, my company actually engaged a vendor to customize an intranet using Joomla. The reason why we had chosen Joomla is because it comes with an off the shelf e-learning package. Even though Joomla itself is a free platform, we paid for the customization of the look and feel of the intranet to mimic our corporate website.
The intranet is hosted within the confines of our company’s server and cannot be accessed from outside.