Somehow, the knowledge that the efforts to tie every trace of my existence together to help marketers target ads to me are done in a cryptographically secure fashion is not entirely comforting.

In general, I have been unimpressed with recommendation engines of any sort. Spotify can't suggest music I'd like worth a damn, and it's working within a relatively specific domain. Whatever fractional gains in ad relevance are currently obtained from this aren't worth the privacy invasions needed to obtain them.