Where did the Yankees audience go?

As anybody attending a game at Yankee Stadium this year can tell you: There's nobody in the stands.

And the Bronx Bombers' TV ratings are dropping, too.

So the team is trying to entice its flailing fanbase by aggressively offering deals on ticket packages.

Sure, the announced attendance is still averaging pretty high this year—over 40,000 in a stadium that fits 50,000—but those numbers are almost always inflated. That's because they count the paid attendance, including season ticket holders, or other people who bought tickets for the game—whether they showed up or not.

As the New York Post wrote earlier this season, a stadium that appeared to have less than 35,000 in attendance earned an announced figure of 45,310—"That wasn't embellishment or even puffery; it was fiction."

In the past few years, even the official announced figure has been on the decline. Overall attendance in the majors has stayed flat in that same period, so the problem is specifically a Yankee one.

But on the other side of town, fans have been increasingly flocking to Citi Field. The New York Mets have seen a big rise in audience this year.

Here you can see in the last few years, the Mets have been closing the attendance gap. In fact, total attendance for New York City baseball has risen—that means the Mets' gain of over 4,000 is larger than the Yankees' decline of 2,400. The Mets have seen a 17 percent jump in attendance since last year.

Since each team moved to a newer stadium in 2009, the capacity of each stadium is over 45,000—meaning neither team is coming close to threatening those levels. TV ratings have seen a similar shift: Ratings for the Yankees are dropping, while the Mets' are going up.

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Local die-hard fans talk about the difference between now and the past. "It used to be nearly impossible to get playoff tickets," said one Yankees fan who wanted to remain nameless. "Now the team is sending emails begging for people to buy them."

Some of the desperate deal emails look like this:

Or this one:

Note a big factor here: The team is forcing people to commit to buying tickets next year if they buy playoff series tickets, with comments like this: Each 2015 postseason group ticket purchase requires an $11 per ticket per game first installment toward your 2016 regular season group ticket purchase.

Even on the secondary market, the Yankees and Mets have seen their ticket prices converge. Data from StubHub shows that a $22 gap in average prices last year has shrunk to $3 this year.

On social media, the Yankees have a bigger presence than the Mets, but that's simply because they have a larger fan base, both locally and nationally. In terms of sentiment, however, Mets fans are happier. According to data from social media analytics firm Brandwatch, 72 percent of Mets tweets are positive (among the ones that can be categorized by sentiment). That's higher than the 61 percent number the Yankees tweets show.

Even though the Yankees fan base is larger by a factor of 4, Brandwatch's data shows the Mets fans are more engaged per person—about twice the average Yankees fan. "With a larger Twitter follower base," said Kellan Terry, a Brandwatch data analyst, "the Yankees should be leading these statistics by a larger margin."

The Yankees have a different perspective on the whole issue, focusing on their position rather than their trend. "We lead the American League in attendance with over 40,000 fans a game," said Jason Zillo, a Yankees spokesman. "We are averaging close to 3,000 more fans a game than the next closest team," he said—again only focused on the AL, rather than all the majors. Zillo pointed out that the team will draw over 3 million fans for the 17th consecutive season, and its YES network will be the most watched in the country.

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As far as the new ticket deals being offered, he said, "good organizations should always look at ways to evolve," and that the team is "constantly evaluating all of our practices ... to provide the best fan experiences possible."

Finally, as far as the Mets go, Zillo didn't look at them as competition directly, saying "it's good for baseball" and the city can support two teams. Rather, he focused on the broader NYC environment. "New York is the most competitive entertainment market in the world. The options are vast and diverse. Broadway, concerts, museums, movies and the like are our competition as much as anything."

Thursday night begins a four-game home stand against the Chicago White Sox. Let's see how many people show up.

—Additional reporting from CNBC's Jodi Gralnick and Jessica Golden.