In the context of the phrase " the free things are the most expensive" I would have thought you will be talking about, water, air, oxygen and the rest of the free things God created for mankind. However in addressing the free things you listed, they are basically not free, these are people's businesses you mentored, for you to use their facility, you have to pay for it, those apps were not invented from the beginning to be use on free bases, for the inventor to build it and put it on the app store, he definitely paid for it to be there, so you the end user too will have to pay for it.
Same here, when I heard that free things are the most expensive, my mind just went to natural resources that we don't pay for, but we use every minute, like oxygen. You know, they say you won't appreciate what you have till you have to pay for it to get it. people that are in the hospital and they have to use oxygen, when you think about the amount they pay per hour for oxygen. When you breathe, you will just thank your creator.
Buy for social media, but then I can't call it free things, if I have to buy data to access it, it's not free, because you already used money to get internet service to access it. The moment I enter these apps and my data is going, the money is somehow going to the creators that entertain me or educate me, whatever I'm doing on social media. So that's just how it works. The social medias are not really free if you must access them with data.