The controversial thing is "targeting" and it's profiling prerequisite. One may not realise the extent of this (say on FB) but have a look:

http://www.wordstream.com/blog/ws/2016/06/27/facebook-ad-tar...

What worries folks is the disproportionate power to influence afforded to private interests. Not just advertising or political campaigns but constant influence on daily choice.

It's an ethical question: should not the direction of society be determined through democratic means? Should we not vote, be equally informed and try to be good citizens? Does the control of information flow undermine this?