Maciej Ceglowski provided more insights into his views about pricing and free services in this podcast interview from January this year (just ff to 24:19): http://5by5.tv/founderstalk/9

His major point is that users regard bookmarking sites as a bank where they want their bookmarks safe and secure. I find that a pretty compelling difference.

So the statement still holds that if you don't pay for a product, you are the product.