Basically, metadata is information about information, something that build and refresh by people all the time, it’s like side effect of online living, all movements has consciences. Is it personal information? Yes. Is it possible to use it? Of course. Idea is, that how to live with this depend only from personal user opinion. If, for example, person doesn’t do anything wrong, not involved in any elicit activity and not feels like a person of government interest, for that citizen is not important if somebody can  collect his metadata and research it. It’s about seven billion people on planet Earth, more than half get internet access. How many people from this number are so important for somebody to be traceable? I think less than one percent. And still if anybody feels like big brother is watching on him, there are two very simple rules in terms of using internet and living pieces of personal information- moderation and discipline.

From the other side, what is better to use during the research : hashtags ,keywords or control vocabulary depends only from circumstances and topic, they all are different tools and represent different technique, and as Badke mentioned, is good to use them all.