[Pasha]

Thing is porn industry was born with web 2.0 concept (which main term is "user-generated content"): see millions of TGPs, linkposts, etc. there like 50-100% of all content generated by visitors (other webmasters) - just porn missing few web 2.0-like features ("folksonomy/tags", auto crowd-moderation, comments etc) due to anti-social behavior of it's primary audience.

I developed few "web 2.0"-like sites (bookmarks, videos, web proxies with social bookmarking, etc) and result was always same: up to 90% of visitors chose "private" profiles to hide their activity from others.

Also content in porn industry isn't too valuable - tons of new movies, pictures, sites being published (and deleted) daily - so there is no point to bookmark certain pages/sites.

EDIT: and one more thing. Can you imagine porn project in mainstream media/news? Like CNN news about Tommy's Bookmarks reach 10 millions visitors daily or CyberErotica get 2 billions visitors. Offline publicity is one of the main supporters of web 2.0 projects - which is completely missed in case with porn sites.