Some speakers continue to insist that the noun data must be used only as a plural, but the consensus is in. Although the singular of data is datum, in nonspecialized contexts, using data as a singular noun is acceptable.
Both usages continue to occur, often in the same publications.
When this data is directly accessible in a database, it’s accessible by attackers.—Forbes
No safety and efficacy data are available for treatment of longer than one year.—Forbes
Paul Brians (Common Errors in English Usage) opines that the usage is so evenly split that “there is no automatic way of determining which is right.” He does advise that writers who want to address an international audience of non-specialists “would probably be safer treating data as a plural.”
In its entry for data, the AP Style Guide seems to come out firmly on the side of “data are”:
data: a plural noun, it normally takes plural verbs and pronouns.
but then offers a back door to “data is” by allowing it to be covered by the rule for collective nouns:
Some words that are plural in form become collective nouns and take singular verbs when the group or quantity is regarded as a unit.
Right: The data is sound. (A unit.)
Right: The data have been carefully collected. (Individual items)
The Wall Street Journal has stated that, as “[m]ost style guides and dictionaries have come to accept the use of the noun data with either singular or plural verbs. . . we hereby join the majority.”
David Marsh, formerly of The Guardian, has compared data to agenda, “a Latin plural that is now almost universally used as a singular.”
Like data, media is a plural. The singular form is medium.
Although the AP guide allows “data is” in some contexts, it really does stand firm on “media are”:
media: in the sense of mass communication, such as magazines, newspapers, the news services, radio, television and online, the word is plural: The news media are resisting attempts to limit their freedom.
Professor Brians insists on a distinction between singular medium and plural media:
Remember that watercolor on paper and oil on black velvet are also media, though they have nothing to do with the news.
Nevertheless, “media is” has been around since at least 1923 and seems to be growing in popularity.
Right now the media is obsessing over Obama’s selection of a Treasury Secretary.—Business Week
You have to understand that the western media is targetting the western readers.—Economist
The best ones know how to convince their team and the public the media is wrong.—Sporting News
A 1923 reference in the OED shows media used with a singular verb:
Mass media represents the most economical way of getting the story over the new and wider market in the least time.
The OED has a separate entry for mass media “with singular or plural agreement.”
The Ngram Viewer shows “media is” overtaking “media are” in 2008.
I’ll give the Chicago Manual of Style the last word on both data and media. Its editors acknowledge that traditionalists prefer to stick to the plural uses, but point out that the new singular uses do exist in a collective sense. They agree that in the sciences, “data is always plural,” but otherwise,
In formal contexts, the most reliable approach is to retain the plural uses unless doing so makes you feel as if you’re being artificial, stuffy, and pedantic. Consider using alternative words, such as information and journalists. Or simply choose the newer usage. But make your play and be consistent—vacillating will not win the admiration of readers and listeners.—CMOS 5:14