Just came across this quote in the first paragraph on page 92 of Chris Anderson's latest book "Free: The Future of a Radical Price". It says this :-
"The Web has become the land of the free, not because of ideology but because of economics. Price has fallen to the marginal cost, and the marginal cost of everything online is close enough to zero that it pays to round down".
Just about sums up why so many web properties (LinkedIn, Twitter etc) are free.