For instance, Yahoo data with Flickr shows that less than 10% of people actually contribute content. So Pareto's rule does not apply there. But only Facebook knows how it works with their users. You can't just blindly apply these "laws" to various user groups and expect them to be even half true.
Think about it in terms of the Tipping Point. Go to the mavens/connectors first. They have all the knowledge know everyone. They'll do the buzz marketing for you and persuade others to join. Another term would be early adopter. I know that I have personal done this for Meebo.
Pareto didn't prove anything, he just made an observation about the italian economy. Other people noticed the ratio elsewhere, and named the rule after him.
But according to this fairly recent article from Reuters http://www.reuters.com/article/internetNews/idUSN17436388200..., the percentage of people that actually generate content is even lower. For example, only 0.16% of visitors to YouTube upload videos. And 0.2% of visits to Flickr are to upload new photos. Wikipedia fares better with 4.6% of visits used to make edits. But it's all a far cry from 20% of users generating content.
So that's about 99.99999% vs 0.000001%.
So beware of creating a site where users have to create totally original content. Sites like hacker news have it easy in that respect.
Sure, in 5 years 20% of people won't be making hollywood films, but in 5 years it could be that 20% of moviegoers make their own movies on occasion.