Re: Users should pay to use Facebook
This is a hobbyhorse of mine, but since someone else mentioned whatsapp I'll point out again that it is an interesting example of something that was intended to be a very low cost pay-for service (until it was eaten by FB - perhaps because its success was setting a dangerous precedent).
The original 'manifesto' of the whatsapp creators from 2012 is still up, and I think it makes interesting reading:
https://blog.whatsapp.com/245/Why-we-dont-sell-ads
When people ask us why we charge for WhatsApp, we say "Have you considered the alternative?"