Would you pay to access Youtube? Facebook? or even Twitter?
Most hot web services work only because they are 'free' for most users, scarcity doesn't help Youtube, Facebook,or Twitter. Think about all the pain caused with every Twitter outage. The early adopters simply get the 'I've been here longer than you' status when you display publicly how long they've been involved with your service.
You can reap other benefits from scarcity besides charging for the scarce product. To take an example from your list, I think "scarcity" helped Facebook a lot in the early days when you had to have a valid email address from one of a small list of schools in order to have an account.
What I had in mind is more the classic pay services, a là 37 signals and the pro flickr accounts. Currently they work by having a free trial or functionality-limited version. I read somewhere to charge the producers, not consumers of content.
