Websites like Facebook and Myspace are shunning the simple advertisement model. Selling private data will become the new standard. A website similar to scope and feature like Facebook will probably not be able to sustain a positive cash flow or break even without selling people's data. UNLESS...they cut features like photo storage, or make a technological breakthrough.
They are making ~$2/user/year (not including expenses). Since it recently supposedly became cash-flow positive, it would be reasonable to assume that the expenses are also around ~$2/user.
The question then becomes, how much of this $2 is slashed off when you decide to not sell private data? You only have to look at the cost of conventional CPM ads (usually less than $0.01 per impression) and multiply it by how many times a Facebook user opens Facebook each year. My bet is that it is far less than 365/2 since Facebook claims 50% of "active" users login every day.