Others have already provided good answers. I'll add my 2 cents: Assume that the companies today are ethical with your data today - they only use algorithmic te

Others have already provided good answers. I'll add my 2 cents: Assume that the companies today are ethical with your data today - they only use algorithmic techniques to figure out what advertisements to sell to you. What guarantee do you have that these companies will be around for the next 20 years? What guarantee do you have that somebody won't acquire these companies when their stocks are languishing - and that the new masters over this data will not abuse it? Who owns Yahoo mail now? Who owns MySpace now? What are they doing with all that data? What incentives do they have to not abuse it? The usefulness of your personal data has an extremely long half-life period. I'd guess it is much much larger than what the average life-span of an internet company.