Which is correct: "All the media is" or "all the media are"?

Actually, when I google for these phrases, I'm seeing this:

• "all the media are" — 110,000
• "all the media is" — 225,000

The Corpus of Contemporary American English paints a similar picture, though the sample size is rather small (1 vs 3 cites). You get more results if you leave out the "all" (445 vs 602). These numbers also include a few cites of the form "the quality of the media is..." or "many in the media are...", which are obviously not relevant here, but the overall trend is still rather clear.

The British National Corpus, on the other hand, favors the plural form. There are fewer results overall, so I took my time to check every single one for relevance, and here's the overview:

                      BNC         COCA        Google

the media are         43           445?
all the media are      1            1         110,000
the media is          19           602?
all the media is       0            3         225,000

So the Americans prefer the singular form, while the British prefer the plural. This should come as no surprise knowing that the British like to treat other collective nouns such as staff, Microsoft or Metallica as plural, too, while Americans prefer the singular. See these answers to related questions:

• Is staff plural?
• Is a company always plural, or are small companies singular?

Media can be treated either as singular or plural. When used in the singular, it is often treated as a collective noun.

The media has gone insane about this trial.

Here is what the OED online says:

The word media comes from the Latin plural of medium. The traditional view is that it should therefore be treated as a plural noun in all its senses in English and be used with a plural rather than a singular verb: the media have not followed the reports (rather than ‘has ’). In practice, in the sense ‘television, radio, and the press collectively ’, it behaves as a collective noun (like staff or clergy, for example), which means that it is now acceptable in standard English for it to take either a singular or a plural verb. The word is also increasingly used in the plural form medias, as if it had a conventional singular form media, especially when referring to different forms of new media, and in the sense ‘the material or form used by an artist’: there were great efforts made by the medias of the involved countriesabout 600 works in all genres and medias were submitted for review.

Merriam-Webster's Dictionary of English Usage (see page 630) also notes the use of media in the singular. It gives examples of it being used as a singular countable noun as well.

. . . partly as a cultural media. -- American Journal of Sociology, 1948

. . . producing a suitable media for organic life. --Britannica Book of the Year 1946

. . . an optical disc media. -- Predicasts Technology Update, 1987

In short, the usage of media as a singular noun is well-documented.

Search results for "all the media is" versus "all the media are" indicate a high level of contamination from false positives—especially in connection with "all the media is." In fact, the first ten matches for "all the media is" in Google Books search results, are false positives.

From Fredric Wertham, Seduction of the Innocent (1955), quoted in Dan Laughey, Key Themes in Media Theory (2007):

The quantity of violence in all the media is stupendous.

From J.V. Vilanilam, Public Relations in India: New Tasks and Responsibilities (2011):

For us it is important to recognize that writing for all the media is an essential part of the training for candidates aiming to graduate in media studies. We start with writing for the newspapers first. Skill in writing for all the media is important because we live in a multimedia world, although priority will be decided in individual countries and their media use.

From Laura Linder, Public Access Television: America's Electronic Soapbox (1999):

The majority of all the media is owned by these ten corporations.

From Eudenilson Albuquerque & ‎Michael Cottam, Polaritons in Periodic and Quasiperiodic Structures (2004):

In the previous chapters we have assumed that the dielectric response of all the media is expressible in terms of a linear relationship between the electric displacement D and the macroscopic electric field E, see e.g. Eqs. (3.1) and (3.2).

From L. I. Medved, "Progress in Pesticide Toxicology in the USSR," in Varo, Pesticide Chemistry–3: Third International Congress of Pesticide Chemistry (1974)

The maximum acceptable daily intake of chemicals from all the media is tabulated and these data are examined for hygienic standardization.

Then we finally get a valid match, from Marc Sedaka & ‎Gregory Rosen, What He Can Expect When She's Not Expecting (2011):

DR. ROSEN: Not to brag, but in my office all the media is stored on a hard drive (no pun intended), so the only thing people have to touch is the remote, which is contained in a sterile plastic bag.

On the strength of that tiny sample, I wouldn't trust the results of a search for that term. The results for "all the media are" have a lower incidence of completely false positives of the type noted above, but they quite often involve instances of "after all, the media are," "above all, the media are" or "...all. The media are," as opposed to completely on-point matches where all modifies media.

The Ngram chart for "the media is" (blue line) versus "the media are" (red line) shows the two phrases moving in rather close parallel over the period 1900–2005, with some relative drop-off for the plural form in very recent years:

But it would be an arduous task to separate the genuine matches from the false positives for each phrase—and afterward, we'd be left with results from a highly artificial environment (the world of publishing) in which editors have for years enforced strict publishing house guidelines (usually but not always in favor of "the media are") that may not reflect the writer's preference.

About the only fairly reliable bit of information that emerges from the Ngram chart is that published writing has not seen a precipitous decline in "the media are" relative to "the media is" even as style-guide condemnation of "the media is" has become far less pronounced. The Ngram chart for "the data is (blue line) versus "the data are" (red line) for the period 1900–2005 shows a similar trend, though with greater separation between the two form during most of the years tracked and with a more significant decline in the plural form in recent decades:

The persistent popularity of "the media are" in Google Books search results came as a bit of a surprise to me—but the only conclusion I feel confident in drawing from the related Ngram chart is that (in published writing, anyway) media continues to be widely construed by some people as a plural noun and by others as a collective singular noun. There may be some few people who switch between "the media is" and "the media are" as appropriate to distinguish between cases of the type that Tim Lymington cites in a comment beneath the poster's question—but I suspect that such individuals are raindrops in a sea of less careful users.

Language and usage evolve, but I’m sufficiently reactionary to regard “media” as plural, regardless of contemporary examples. In regard to the Fourth Estate, “media” to me signifies the print medium, the broadcast medium, et. al.

Of course, this comes from one who occasionally uses “Yr. obt. svt.” as the complimentary closing on my correspondence...