The best software which may fulfill your requirements is Trillian, filled with features, user friendly/enhanced UI, free (premium if you need some extra features).
Facebook - Yes
IRC - Yes
Skype - Yes
Whatsapp - No
I will come to the Whatsapp later, but as far as the features go for Trillian its packed with the ones you would expect from a decent "all in ...
You can use Miranda IM:
free and open source
supports IRC, Facebook (via plugin) and Skype (needs running official Skype client)
over 500 addons
Regarding Skype, this is issue is that Skype retired their desktop API in December 2013... (official M$ post)
WhatsApp can actually be configured to use much less phone space:
Go to the settings
Touch "Data and storage usage"
Find the "Media auto-download" section
In each of the items in that section, unselect everything
Delete everything in your WhatsApp/Media folder.
Now, WhatsApp should take no more than a megabytes or two, rather than hundreds of megabytes.
There is a python WhatsApp library called yowsup that should let your RaspberryPi act as a client for a WhatsApp account. See here for a comprehensive walk-through of doing so.
This library will definitely allow you to receive messages, with originator details, on your device.
Threema would fit your requirements almost completely:
Isn't owned by Facebook, inc.: Nope. It's owned by Swiss-based Threema and has a strong focus on privacy. So it's just the opposite to those "kraken"
Is gratis to use: Using it is gratis, yes. There's just a very reasonable one-time fee for the license (less than 3 bucks). Per device that is (you can ...
Microsoft Office 2016 provides the "Spreadsheet Compare 2016" tool (you can find it in Windows Start Menu). It can compare Excel files. So, you can open your CSV files in Excel, save them as .xlsx and then compare using the mentioned tool. Also, you can use just the Excel itself to compare two tables by selected columns (VLOOKUP function can be helpul).
Since in the edit you mentioned you're open to suggestions other than Whatsapp, then try using Kakaotalk. It matches your requirements in the following:
Has PC and mobile versions (Windows, OS X, Android, iPhone, Blackberry, Nokia)
Does not require registration, only a phone number
Is not associated with Chrome or any Google service