prepared to pay
The problem with trying to work out what people are prepared to pay to use a social network is that the value goes down if less people use it. So I put up with facebook because all my friends are on there, some of my friends might be prepared to pay to stay on facebook some wouldn't but if half my friends that weren't prepared to pay left then the half that were prepared to pay would no longer see the value in paying for the service.
If you are trying to start a service from scratch and you want to charge people you aren't going to attract anyone because nobody else is using the service.
It turns out (friends reunited) that charging people to use a social network doesn't work.
Of course if there is an actual product for sale then you it is possible. I pay for a flickr account, but for that I get unlimited space to store full resolution versions of my photographs together with the ability to download them as required.