This is a factor you need to take into account, but in my experience, people overstate it. It's not actually that hard to get people to pay for things online.
The way you do it is the way it's always been done historically: offer a value proposition that justifies the money. And don't offer it for free at the beginning. People rightfully get very angry if you change the deal after they've come to rely on your product or service.
I don't know, my experience is that there is a group of people who don't mind paying for things online. I can generally group them in "people who own a mac, an iphone or an ipad"
I still am clueless as to how to generate a network effect without giving things away for free. And as you said, once it's free, it's expected to remain free
The cynical me says that some people are able to generate a network effect because of fame or because of an existing network to which they belong (i.e. being chosen at YC or published in a newspaper). For example, FB was promoted to death in all US campuses for free during the 2000s. I wonder how that happened (honest question)
The way you do it is the way it's always been done historically: offer a value proposition that justifies the money. And don't offer it for free at the beginning. People rightfully get very angry if you change the deal after they've come to rely on your product or service.