The measurable and obvious aspects of the modern mass media are things like newspaper and book sales, television and radio usage, the number of postings on the internet or usage levels of social media.
Even more validly, the mass media can be seen in what people talk about.
But the mass media is actually what happens inside people's minds - the mass media is fundamentally a matter of what people think about.
And the above are merely indirect ways of detecting and measuring what people think about and how they think about it.
What people think about is assumed to affect their attitudes, knowledge, motivations - in a word behaviour - influence, that is, things like what people buy - how much and of what type, what they spend their time doing, what they put their discretionary effort into doing....
But the actual core business of the mass media is only indirectly knowable by such things.
A consistent pattern of observed phenomena - when very large numbers of people are buying, talking and seeking information non a specific topic, may be the best evidence we can have about what really is going on in the mass media - but it still isn't the thing itself.
Where the mass media is really happening (or not) is in minds - the minds of the creators of mass media, and the minds of the consumers of the mass media (which can be the same mind - since all mass media creators are also, and mostly, consumers).
In sum - we can detect and measure communications; but not that which is communicated.