All This Is Free?

Many sites on internet offer us free of charge when we use their service, such as email, blog,web hosting, etc.What is behind this virtual marketing strategy?Because as we know that nothing in this world is free.This is a common strategy from many virtual marketing sites to invite internet users to joint and revisit their site to gain more popularities and traffic.The more traffic, the more money will be gain from advertising or buyer or donatur.So I do not think if the future of a radical price is free as said by Chris Anderson.Because I say again nothing in this world is free.