The idiom “be a thing”

I'd guess this fairly recent use is related to the rise of internet memes, and whether an item is widespread or just someone's own: "Is this a thing? Is this a trend?"

It dates back to at least 2006 on blogs and 2001 in film and television.

The second Urban Dictionary definition is relevant:

2. A Thing

An action, fashion style, philosophy, musical genre, or other popularly recognized subsection of popular culture. Normally used in surprise at it's existence. Becomes official when a wikipedia article is created for it.

"Whats the deal with those skin-tight body suits?"
"Oh, you mean Zentai?
"Huh. I didn't know that was a thing."

[thing] [something] [noun] [unknown] [popular] [pop culture]
by mrpeach32 December 29, 2011 82 up, 18 down

2006 blogs

It dates at least from 2006. Here's a March 2006 blog comment:

Sifu Tweety said,
March 29, 2006 at 22:16

A) How did the skull get in your ass? Is this a thing? Are there websites? Please don’t tell me about them.

And a blog comment from April 2006:

Good god, it's like people lie in wait, pressing refresh, waiting to comment. Is this a thing?

Posted by: Meg | April 03, 2006 at 10:48 PM

Here's a one from November 2006 on the Guardian's film blog:

In the changing room of the gym, there are a large group of twittering women standing around. I'm mainly blocking them out, but suddenly, the whirr of the hairdryers dies down, and one phrase leaps out of the burble...

"...and you know, you'd think it would be really good. Because it's got Vince Vaughn in it."

What?

Is that a thing? Is it really? Is it a thing that people would naturally assume, seeing Vince Vaughn above the title of a new release?

The Straight Dope Message Board covered "Is this a thing?" with one saying they first heard "Is that even a thing?" on Seinfeld (1989 - 1998) but I couldn't find it. Another said they'd heard "Is that even a thing?" on Family Guy but I couldn't find it there either.

A Lingua Franca post on "I Guess ‘It’s a Thing’" found a 2001:

The distinction has applied through the history of the phrase; and I’ll note that the “existence” meaning is an elision of the traditional “(there is/is there) such a thing as…” The first occurrence I’ve been able to find was in a 2001 episode of That ’70s Show, involving a pick-up basketball game:

DONNA: Oh! That’s 16 for me and Hyde and four for the losers! You guys ought to get a mascot … a big, green, furry loser!

ERIC: That’s … That’s not even a thing.

But that seems to have been an outlier, with the next use not occurring till February 2007, when the LAist blog asked: “Is jazzy folk even a thing?”

Finally, following on and searching more television and movies, here's The Adventures of Pluto Nash (2002):

00:03:41 No, a kilt. That's how people know who I am.
00:03:43 People think of Anthony Frankowski, they immediately think ''Scottish crooning.''
00:03:48 There lies your problem. That's not even a thing. Did you make that up?
00:03:52 Yes, I made that up. It's a thing if I made it up.

from Online Etymology:

thing (n.): Old English þing "meeting, assembly," later "entity, being, matter" (subject of deliberation in an assembly), also "act, deed, event, material object, body, being," from Proto-Germanic *thengan "appointed time" (cf. Old Frisian thing "assembly, council, suit, matter, thing," Middle Dutch dinc "court-day, suit, plea, concern, affair, thing," Dutch ding "thing," Old High German ding "public assembly for judgment and business, lawsuit," German ding "affair, matter, thing," Old Norse þing "public assembly"). Some suggest an ultimate connection to PIE root *ten- "stretch," perhaps on notion of "stretch of time for a meeting or assembly."

Used colloquially since c.1600 to indicate things the speaker can't name at the moment, often with various meaningless suffixes, e.g. thingumbob (1751), thingamajig (1824). Southern U.S. pronunciation thang attested from 1937. The thing "what's stylish or fashionable" is recorded from 1762. Phrase do your thing "follow your particular predilection," though associated with hippie-speak of 1960s is attested from 1841.

I can't do better than this. Lucky find as it was.