That would be kind of cool, actually. A $5 one-time fee for every user. It would keep out the riff-raff and spammers, and there would be no need to sell user content or even have ads at all.
That may be a bit low. Facebook makes approximately $5/year off of each user.
Another problem with a $5 one-time fees is that people expect to use your service in perpetuity. That price point probably isn't sustainable over a more than a few year period.
$5 per user is equal to $5,000,000,000 for revenue and I think Facebook made $1,000,000,000 net profit last year, so Facebook makes about $4.5 per person gross and about $1 per person profit at the moment.
