You can share it, of course - but, how would they correlate it with data from other websites? How can they tell that a user on your site and one on my site are

You can share it, of course - but, how would they correlate it with data from other websites? How can they tell that a user on your site and one on my site are one and the same? They cannot (exceptions noted above), which makes it rather useless for profiling a user